Well, a number of different cases, all of which are pasteurized before use to ensure that there are no bacteria in it to form mold in a humid environment.
Coffee is a popular bedding option, especially among home mushroom growers, as they are readily available in smaller quantities.
Large coffee chain coffee shops are available for free in many parts of the country. Simply pasteurize these coffees, and you have the beginning of a mushroom growing environment.
Hardwood dust, coconut mixed with vermiculite, and various fungal fertilizers (although usually horse and poultry) are also used in the mushroom compost base. Other products used include peat moss, rice or cotton cardamom, canola or soybean meal, and more.
These materials not only compost to create a quality mushroom substrate, but also later create other excellent gardens and add a lot of organic matter to your soil.