They can't be found growing on trees in the woods.
Many growers raise the mushrooms on hardwood logs inoculated with the spores.
Oaks are the most reliable wood for producing mushrooms. Canning grade paraffin wax works well. Logs are watered to keep them moist, but allowed to dry between waterings. If logs develop a green or blue surface fungi, discard. Logs seldom produce mushroom before two to three years of incubation outdoors.