D&D has no world outside of meta-influence

Allow me to frame-challenge here: for all intents and purposes, there is no canonical D&D world outside your DM's representation of that world at the table.

Your DM may have some information prepared (campaign notes, established facts from the campaign to date) or even official information about the world (a campaign setting book), but a fundamental principle of DMing is that none of that is effectively canon until the DM describes it at the table.

Contrast this with a video game, Skyrim for example, where the game data records every detail of the city of Whiterun, down to the name of the man who runs the general goods store, what goods he has, his level of stock, and price of every individual item.

A single human DM can't realistically prepare and track every detail of the world that the players can explore. Skyrim took an entire professional design team years, and the entire game only covers an area of 14.3 square miles containing only 1,144 NPCs. Skyrim's city of Whiterun has fewer than 100 NPCs, while D&D's city of Waterdeep alone had 132,661 inhabitants in the year 1372 DR, with an area of 5.74 square miles. For real-world comparison, England is 50,346 square miles.

As a result, a DM is fundamentally expected to invent new details to the game world in response to player questions or actions. When a player asks "Is there a wizard's guild in this town?" the DM does not necessarily consult a sourcebook, but invents an answer on the fly based on what would be reasonable and what would further the story. Using this to portray a believable and consistent world is a fundamental DM skill.

The flaw you're describing is a DM who is too convenient, in a way that strains credibility and breaks the verisimilitude of the world that the DM is tasked with portraying in a realistic and plausible manner. Like most DM problems, that is fundamentally something where you just have to provide feedback to the DM so that they can improve their art.

Check in with your DM

You've got the crux of this right in that you're simply going to have to check in with your DM on what general world knowledge you have and don't have and whether or not the backstory you're developing will work.

It's great that you want to develop a backstory and have desires for items that will drive your roleplaying and the mechanical usage of your character -that's a DM's dream :) In order to best do this, just continue working and talking with your DM about it. The worst thing they'll tell you use is either "no" or "slow your roll, you don't need to go there". They'll definitely be able to help guide your character creation so that it you can build and enjoy playing your character in the world the DM is making for everyone.

Don't think of this as "metagaming". You have a character and you want it to do certain things. What you'll need to do is act appropriately in-game with the information the DM has given you out-of-game. Or they'll be able to work with you on items you need/want to make sure the world they're developing works with the character you are creating.

When I either play or DM, I always like to talk about my plans. As a DM, I've got the storylines I write, but I also want my players to have character arcs and their own stories - and they're the ones who are going to help guide me in developing those. If I'm a player, I want my DM to know the things I'd like to explore, even if it's far into the future.

The specific ranger question

For this, you'll just need to ask your DM if component pouches are easily purchased. They'll let you know how it works in their world and what it will take to get it.

Magical Items

These are bit different and every DM will treat the presence and availability of magical items differently. But either way, I'd definitely talk with the DM as to what items you're interested in.

In my 5e games, we've generally played with magical item random tables and little access to 'get' what we want. And I can tell you that it wasn't all that enjoyable. Getting random items that you can't use and have no way to sell or trade was underwhelming. Personally, when I DM I try and work with my players to get them things they're interested in or to provide utility items that let them explore fun ways to solve problems.

Suggest to your DM that they roll for likelihood

In a game that I'm playing in at the moment, my DM, if ever confronted with a decision about some detail he hadn't considered before now (no matter how trivial, literally whether this nameless guard had cornflakes for breakfast or whatever), will then roll a d20 (with some arbitrary DC in mind that he never tells us because, honestly, we never asked) to decide whether or not this detail is true or false.

In your case, whether "Ye Old Magic Shop" happens to have a Component Pouch could literally be decided with a dice roll, such as odds it does, evens it doesn't. That way, you haven't "meta-influenced" the world so much as prompted the DM to use random probability to decide whether or not the item would have been there regardless of whether you had asked or not.

Of course, what my DM does isn't in response to someone having concerns with "meta-influencing", but it effectively addresses the issue nonetheless*.

This still requires a conversation with your DM

Ultimately, there is something that is bothering you and this means that you and the DM (maybe the whole table?) might want to have a chat about to see if they a) agree that it's a problem that they care about beyond just telling you to get over it, and b) that my proposed solution (if you like it, or otherwise some other suggestion that you've found that you like) would be something the DM would be willing to try out. After all, the DM is under no obligation to do so.

* Meta-influencing is inevitable and unavoidable

Even my solution (or rather, my DM's solution) can be criticised as being a result of you asking about a Component Pouch or whatever, and that the DM would not have rolled anything to decide whether "Ye Old Magic Shop" would stock one. Effectively, your asking the DM about that item simply increases the chances from 0% to 50% (or whatever) instead of 0% to 100% (i.e. the shop suddenly has one!) that the shop now stocks that item.

Ultimately, this is unavoidable. If the DM didn't think about it beforehand, then there is only one of two ways they can respond; either they say "well, I never thought about that before now, so no, there are none in my universe, tough luck!", which isn't fun, or they are going to find a way to insert it in, in which case you've "meta-influenced" the game. This is what I mean by unavoidable.

It's not really a response to your asking so much as a response to the DM not being capable of taking literally every single possible thing into consideration as they create the world, since that is impossible. On some level, you will have to accept that the world will shape around your questions as you ask things that the DM has not thought of yet.

In addition to how the DM builds the world, as outlined by the other answers, there is a difference between what your character knows about the world and what you know about the world. For example, if your character has grown up in the area, she probably knows that Crowdedville is about a six-hour walk from Tinyton.

Asking your DM a question about the world is in some sense asking if your character knows anything about that thing (or at least that's how I handle it when I DM). For example, asking about finding a component pouch in Crowdedville is implicitly asking the question, "Does my character know anything about where she could find a component pouch in Crowdedville?" At that point, your DM follows the ideas laid out in the other answers determines if it makes sense in their world to have there be a component pouch in the town, and if so, where it is and if your character knows about it.

Order the items from the shopkeeper

This answer is only helpful if you have a hub you frequent, but have you considered placing a request with "ye old magic shop" to get you that item? Maybe you'll feel less bad about it suddenly appearing in the inventory if you specifically ordered it from the shopkeeper and he/she/it has it the next time you come by.

Admittedly, this only helps specifically with items you need/want. Maybe you'll even want/have to pay premium for getting a specific item but component pouches shouldn't be too hard to come by depending of course on the world the DM wants to create. But that is something they get to decide anyway.

More generally if you want to know something about the world you could ask the "NPCs" that you feel would know about it. If you are in a new town you could go to an inn/tavern and ask the barkeeper (since those types of people generally know about the town they live in), "Hey I'm looking for a component pouch. Could you tell me if this town has a shop that stocks those?".

Talk it out.

I had a tool kit that I wanted so I discussed it with my DM out of the game. A good DM will discuss a realistic scenario where obtaining that item makes sense in the story.

In my case I just had to ask the shop keeper and they “had one in the back”. The DM said that one of his goals is that people have fun and if he can make an idea work than he is all for it.

Personally I have struggled with the same problem in wanting to research and know more about the world of D&D but not wanting to drive the story toward what my research has shown. I have found it helpful to actively put myself in my characters shoes and remind myself what they know, much like a writer will get in the head of a character so that their interactions and choices are cohesive and make sense to the story as a whole. I work backwards if I am finding I want to go a certain direction with my characters development or story. I ask if my character would even want what I (out of character) want, if the answer is yes than I ask how would he go about seeking that out, what questions he would ask to seek his desires. Would he give up easily? Would he get distracted by a bar fight and forget about it? This has worked pretty good for me thus far and I'm still seeking the discrepancy between my character and myself. As a work of discipline in imagination this process is often as enjoyable as researching the best weapons and gear.

There are a lot of great answers here, particularly Quadratic Wizard's and NathanS' (the former of which is the answer I would give as well, had it not already been written). But something which might enhance the elements you're looking for is to simulate the process of your character searching for information, goods, etc.

If you request that the DM help simulate hunting for an item, there are some published guidelines that may help. I'm thinking of something like a variant on finding a buyer for a magic item, but in reverse. I've done similar things in my games, though I don't necessarily recommend doing so (see note below).

So if your character wants to find a component pouch, you roll some stat (it might be survival if looking for an herb in the wild, intelligence if looking for a manufactured good in a city, etc.) against a difficulty set by the rarity of what you're looking for and the size/complexity of the area to be searched. Finding a component pouch in a large, magically active city might be DC8, while finding an unusual material component in a smaller, satellite city might be a DC17.

Note:

I don't quite recommend doing this, because while it can enhance the grittiness and reality of the setting, there are some mechanical issues that cut against that effect. Here are some observations I have made when trying to simulate these sorts of activities at my table, using the technique outlined above:

First, time must be a factor. If you have unlimited time to spend searching without any consequences for delay, then the upshot of this approach is that you waste time rolling until you reach a successful one, which you always will (eventually). Time is often poorly represented in D&D, and it's hard to introduce it in this narrow context but not note its lack elsewhere.

This also makes it awkward for uninteresting tasks, like finding a common item to buy-- it's less interesting to dedicate your scarce in-game time to that rather than the actual adventure you could be having.

Second, this approach means that you have to be ready to accept failure or incomplete success. Hunting for a component pouch suggests that they aren't that easy to find (otherwise it really would be as easy as stopping into the first magical supply shop you encounter and finding one there), and as long as you don't have unlimited time you might not find the thing you want. In the case of something like material components, that can be a serious problem: your magic won't be an option for you without them.

No one can decide for you which would be more fun between a game where you search for components and may have to do without magic versus one where you assume components and get to use magic.

Finally, there is not enough throughput to really simulate things to this degree. Your DM does not narrate every single feature which must exist in a setting, choosing instead to focus on major pieces which illustrate the scene and/or are relevant to advancing the story. When your character enters a shop, you don't hear a list of every single item that such a shop would carry-- that would take ages, and knowing about how many colors of sealing wax they sell is not relevant if you are never going to use, encounter, or in any way interact with sealing waxes.

This usually gets abstracted away. When you tell your DM that your character wants to find a component pouch, your character looks around town "off camera" until they encounter a shop that has what you want. You, the player then get a report that your character has found shop X that sells component pouches for Y gold pieces. If you want narration to explicitly express the search, that's the kind of thing to mention to your DM ("you spend hours wandering the city, looking through market stalls and making discreet inquiries. Eventually you come across a decrepit-looking shop with no shingle, but you feel confident that you can find what you need there..."). If you want to play out the search, you would still mention it to the DM, but you are really asking them to invest vastly more effort on content that is specifically not fun or exciting.

For the example you give of securing items, the listed price for an item in the game materials gives you an idea of its relative abundance, unless the DM says otherwise. In the case of 5e, the "Item Rarity" is actually listed. So, until your DM says anything about it, you as the player should know that a Component Pouch is "Standard", and thus you should not be surprised if your character finds it with ease. Just because your DM never mentioned them before doesn't mean they weren't there, your character just wasn't looking for it.

On the other hand, if the DM is presenting you with exhaustive stock catalogs for each vendor, then I would suggest they are shooting themselves in the foot and doing too much work. You could talk to them about giving less specific detail so that it is less jarring when they have omitted some things that should be relatively abundant.