I don't believe I've ever had miso before, so substituting for something similar is NOT important. what is important to me is, what will go WELL IN IT'S place.
Here is the recipe:
12 cremini mushrooms
1 Cup pine nuts
2 garlic cloves(or more to taste)
1 Tablespoon low-sodium yellow miso paste
1 Cup fresh basiil (about 2 ounces)
Juice if 1 lemon
Remove mushroom stems, put aside. Puree pine nuts, garlic, miso, basil, lemon juice and fill in mushroom caps. Bake at 350 for an hour until soft.

Why can't you just use miso?

I can't find any in the stores around here, and I'm wanting to make these tonight. I've looked for three days at all the stores in the area with no luck. oh well I tried

Use veggie boullion. My absolute favorite is Better than Boullion NOT beef, or chicken, base, or the vegetable kind. It's much less sodium than most. Be sure you double check the jar because they make real chicken and beef bases. vegan is green, but otherwise looks similiar except for the 'not".
What is this recipe for? It sounds like a spread, and could leave out miso entirely. Maybe a drizzle of olive oil.

The recipe is for stuffed mushrooms. It seems like the miso mixture is to make some sort of pesto or somthing to stuff in the mushroom caps

You can just leave it out--the flavor will be different, but I think with the other ingredients you should be ok. Miso adds a salty, fermented flavor that is kind of cheesy. You might want to add some extra salt, since miso is really salty.

If you have a local asian market, you should surely be able to find miso there. Also natural/health food stores might have it, but it'll probably be more expensive. I usually pay $5 for a 12 oz tub, but the "organic" varieties can be $8-12 for the same amount. Miso soup is super easy and delicious, if you can get your hands on some miso.

Thanks for the suggestion. We don't have any asian markets around here that I'm aware of. Our walmarts and krogers have an asian area, but there's not much, and there was no miso paste of any kind. The only health food store here did not have any either. I will add saly, then taste it and see if there's anything else i could add that might taste good.

It won't help you for tonight, of course, but here is some miso you can buy online for a very reasonable price: Roland white miso, unfortunately it's a super huge amount. Maybe you have a friend you could split the order with (it's a 2-pack)? It will last for years in the fridge.

If you want to get a smaller amount, you might ask the health food store if they would special order some for you. The inexpensive brand I buy is Cold Mountain.

When I read one comment that described miso as "salty and cheesy," Parmesan cheese came instantly to mind, and it would be good with the other ingredients in your recipe. It will change the character of your recipe from Asian to Italian, but it'll do. I know I'm answering this long after the night you wanted to make this dish, but perhaps it will help someone else.

I think its as hard to find a vegetarian parmeseam than miso! I have heard of it, it's almost always with animal rennet, even from companies like Sargento which uses plant rennet in all other cheese
I don't miso tastes cheesey at all, but I do get the sharp edge that, yeah, could be like parmasean. Now I'm thinking a bit of green olive juice with olive oil, or veg broth

Looking back, I'm not sure why I didn't recommend nutritional yeast. It seems so obvious now. Nutritional yeast is my vegan substitute for parmesan too.

There we go! But if you can't get miso, would you have nutritional yeast?
i'm sticking with veg broth or something like soy sauce

Tahini can be subbed for miso, too.

I can see that, but that seems like it could get a little thin. Maybe another nut butter, with a little soy sauce mixed in to boost saltiness and umami.