The majority of medicinal mushrooms are grown on sawdust. This sawdust is a byproduct from local sawmills. Mushrooms are great at recycling waste materials and this is a perfect example. Sawdust that might normally be thrown away or burned, can be used to grow mushrooms on.
One exception to this is reishi, which is grown on individual hardwood logs in the ground. These logs come from locally sourced beech wood.