Hed Fang เห็ดฟาง
Straw mushrooms are as common in Thailand as white button mushrooms are in the US. Straw mushrooms are meaty and mild in flavor. They cook well in soup such as tom yum (spicy and sour soup). While I've never seen fresh ones in the West, canned straw mushrooms are available at most Asian markets.
Generally, since for most dishes that use these mushrooms, the straw mushrooms themselves are not the driving flavor, you can substitute more easily found, locally available mushrooms.