Horticultural medum grade perlite.
Here is a wonderful document on the use of perlite in mushroom cultivation, from the Vaults of Erowid:
We recommend boiling and straining or draining this product prior to use as a humidification aid. This helps wash away the perlite powder and increases efficiency in moisture release. The one gallon sack should be enough perlite to lay two inches deep in a medium sized mushroom terrarium (10 gallon containers). For larger mushroom terrariums, you might need two bags.