Either buy them in an asian super market or grow them on your own. (Photo below).
How to prepare: add them to your skillet, put a splash of how water in it and put the lid on. Cook them for a few minutes, then lid off, and some (I prefer A LOT) of butter and spices. Sometimes, depending on the kind of mushroom, only salt and pepper (high quality ones), sometimes onions or garlic, whatever you want.
Prepared dish (Pheonix oysters)
The same Pheonix oysters I grew
Brown Shimeji on my balcony in a tub (cloned from the asian market)
Lions Mane I grew
Oyster mushrooms in a big bag on my balcony