Mushrooms grow in shit. How sterile do you want it?

I mean, I’d get a bag of the same stuff you’d get for a vegetable garden. Also, be choosey with any fertilizer. You don’t want to be adding chemicals to something you’re gonna ingest.

I’m betting you can find some good books on gardening and mushroom farming at the local library (if those exist where you are). You’re gonna get a MUCH better result in anything you do if you learn about it instead of just winging it.