Coffee grounds

Coco coir is very cheap and is what I use. If you want more of a project, you can make the inoculation jars and sterilize the grain yourself. That way you’ll be taking a spore/liquid culture syringe from a tiny blob of mycelium to a whole network of fruiting bodies. Doing that will also be much cheaper in the long run if you stick with the hobby

Mycelium has been shown to colonize some of the most unexpected substrates - cigarette butts [1], sawdust, you name it.

https://circulareconomy.europa.eu/platform/en/good-practices...

The thing with microbes is not if they can grow in a place it's whether they can get there first.

Beer is basically knocking out natural bacteria and trying to get yeast growing before the bacteria can turn it into cleaning supplies. The alcohol is kind a there because it kills bacteria.

So for instance I put winecaps (Stropharia rugosannulata) into wood chips that had already been exposed to the elements for six months, and ended up with more than I could possibly eat.

Meanwhile oyster or shiitake mushrooms want a fresh log, cut with a sterilized blade, and cross your fingers and hope. I haven't even tried because I've watched people who know way more than me about mushrooms, fail.

I think I have some logs that might have lions mane in them, but they're fighting the turkey tail that was already in my local environment and also on the property of the person who donated the logs.

Certain species do better in different substrates, but for the ones I've grown coco coir (also suggested by holly01) works great. There are some additional bits you can add to improve results but it starts there. You can hydrate it with hot water in a 5 gallon bucket. There's lots of tutorials on YouTube.

Depends on the species. For something easy to grow like oyster mushrooms, straw. Do decontaminate the straw. Cooking water or hydraulic lime water should work for that.