Caveat emptor--check your goods before leaving the stand. Last year, I bought fresh porcini mushrooms ($$$), got home and discovered most were well, well past prime. When I mentioned it a few weeks later, there was no interest in any gesture, verbal or in goods, to make it right.
I know it's a farm stand and they don't give receipts. But I wasn't going to hold on to the rotting mushrooms for a week or two until I came back to the JFX Farmer's Market. It would have been nice to see some sense of pride behind their products. I still buy from the stand--but now I inspect all the mushrooms in the bag before I walk away.
Ferial Welsh is the Mushroom Lady. She is also at a couple of Freshfarm's markets in the DC area. Their Dupont Market is every Sunday year-round.
Here's an article profiling her and includes a nice video clip:
The Freshfarm site includes information on where in Pennsylvania her mushrooms are grown:
The Pennsylvania farms: