There are many kinds of mushroom powders. I've probably used 7 types in all. But as I have very limited stuff at hand, I will show you only two of those.
The one on the right is my husband's favorite.
It goes in an airtight plastic container and I know "Knorr" will come to your minds when you stare at the logo. I actually believe it is a product of Knorr but I'm not so sure. But if it is, it's probably good because it's an old, big and trusted company, isn't it?
The picture below it (lower right) is just the 500g version, twice the one in the bottle (250g).
This one below was a product I once bought from a Filipino friend who tried to start a business in the Philippines.
I like it very much because 1.) it's confirmed pure vegetarian; 2.) the product name "Shiitake Dashi" is written in Japanese 「しいたけだし」 ; and 3.) it's not wallet-painful, specially for starters because it was just Php80.oo per 60-gram pack.
If you want to check out these products and many other mushroom powders, here are links to the stores where these may be bought:
Wabi-Sabi Noodle House and Vegetarian Grocery | map | Facebook | blogger's photo of Wabi-Sabi's grocery rack
Happy Veggie Health Food | map | blogger's photos of Happy Veggie's grocery rack
Quan Yin Chay Vegetarian Food Garden | map | blogger's photos of Quan Yin Chay's grocery rack
If the above places are also inaccessible to you, you can check out the mushroom powders I saw sold online. I haven't bought from them, but I'll probably resort to them in the future. Check it out.