I live in a small town and do not have access to some ingredients - what would you use to replace black vinegar and the mushroom seasoning? Thanks

It won't have quite the same flavor, but you could try replacing the black vinegar with rice vinegar or rice wine vinegar.

In place of the mushroom seasoning or maggi seasoning, you'd want to use something with a nice and strong umami flavor. If you are already getting dried mushrooms for the recipe, you could add an extra mushroom or 2 the hot water you're soaking them in, and then cook the soaking liquid down into a concentrate! Might give the dish a little extra punch of umami.

If the ingredients you are seeking are shelf stable (and not just the two you asked for here), there's no reason not to consider online retailers.

And there is nothing stopping you from asking your local mom-and-pop stores to place a special order. After all, most of this stuff probably ships from the same wholesale warehouse that supplies the nearby big city restaurants. You just need to plan ahead and ask.

You can use rice vinegar and maggi seasoning as substitutes.