I guess I'm wondering if there's companies with their own specific websites who specialize in making game assets and selling them.

Not many, this isn't very profitable. Most artist will sell some stuff on popular sites or on their blogs.

A artist isn't some kind of on demand asset store, all of us know how to both make and implement our art.

If you ask a artist for a character that can run and jump, they will make the sprite, program the running and the jumping and place it in the game for you. Because how the character moves and responds, is part of the art.

So getting to the point, all assets on the asset store is defective products:

We sell them knowing that they wont work as intended and the buyer knows they have to modify the art to work with there game; they will be doing half of the artist's work. They also have to share, and take the risk that it doesn't work.

Assets takes a very long time to make, selling on a market doesn't guarantee enough sales.

No artist wants to focus a huge amount of time making something that won't work as it should, so most market assets is low quality or just some random piece a artist made as training.

A artist also can't charge full price for a market asset because of the downsides, as such the unspoken rule is to sell at 10% of the usual worth.

Most developers are amateurs when it comes to art, so even if a artist spend days making a good piece, it will only look cluttered to a inexperienced user; so most art on market stores are basic low quality stuff.

However the most damming part of market assets is the fact that instead of expanding the art to work with the game, you will have to change the game so that it can make use of the limited functionality of the market art. (Unity games is a good example of this.)

For these reason the demand for market assets are low, a new developer will use them a few times ,then move to using a artist when they can afford it.

A artist makes around the same amount of money, from one asset in 2-3 months on a asset store as they would if sold to a customer. They could also sell the full intellectual rights for more than double the amount a asset would make on a market.

So in short, making market assets is less profitable to an artist.

I feel the same way regarding the lack of quality of for topdown 2D tilesets. So far, I've looked in places such as opengameart, gamedevmarket, etc.. and they're generally not as good as the non-2d assets you see in unity store. I think it might be because more people are working on modern style art assets nowadays instead of the retro look that you posted.

What Scouting Ninja said, really...

add to that the fact that Tilesets do not work well together unless ALL of them are done by the same person in the same style... or at least the artists working on them would agree on a style and try hard to keep their tilesets compatible.

So unless you find an artist that actually does EVERYTHING you need (which, even for a basic RPG Project, can amount to quite a lot unless you want all the areas in the game to look more or less the same, without any setpieces), tilesets will not combine well, and are more of a starting point to work off (like showing you a way how to organize your own tilesets, or have something to show a artist as an example).

That is true for ALL assets, 3D included, which sadly do not work well together more often than not, see all the low quality Unity games populated by mismatched 3D assets bought from the asset store. But I feel nowhere will it be more evident than with 2D tilesets.

Really, you should look into your options: either downscale your scope and ambitions so you can create your own art, or find an artist to work with and some money to pay him.

Stock art is only a good source for simple cookie cutter assets needed to fill the world, and only really for a realistic 3D style nowadays. For a more stylized style, you will never find assets that fit the exact style you are aiming for. And really, art is an important aspect of your game, you should try to not shortcut it too much. You also usually don't just copy and paste a games story, right (knowing that this is how many games end up with bad storys in Unitys/Steams bottomfeeder hall of shame)? Finding ways to short cut what could be way too much to create for even an expierienced artist is a smart way to do game dev, but buying stock assets should only be one of many ways of achieving this.

One thing to note is that art does not have to be that hard, impossible to do task many non-artists think it is. There is no magic behind it, just a lot of practice. And you can achieve a satisfying result with less practice, see the pixel-retro-wave, and some of the more artistic graphic styles like the one of Limbo.

Not saying it is "easy" to create a good look with a very stylized style like that... just that maybe you need less practice as an artist to start producing stuff that starts to look "right".