Our small grocery store, one of only three locations in the "chain", carries a line of spices/herbs/mushrooms from The Nutmeg Spice Company
. I remember pacanis mentioning buying something from Nutmeg Spice from his local (PA) grocery store. Wondering just where their product is sold, I looked at their website and saw they are available in 48 states! I checked a few general cities from DC member locations and I'm guessing most of you guys have a store nearby that carries their line! Seeing as how their line offers 10 different dried mushroom varieties, it might not be too hard to hunt down what you need.
Maybe if you find Nutmeg Spice products in your store but they don't have the exact item you're looking for, you might ask the store manager to get it in. I know the corner store started out with a small rack offering about 10 items. Now they have two large "bookcase" shelving units and probably have every product Nutmeg sells! I know I've always been happy with anything I've used over the years. I'll still pick up a jar from the corner (3/4 miles away) instead of driving to Penzeys (45 miles) for even a couple spices!
By the way, these are sliced mushrooms. To get mushroom powder, all you have to do is whir the stems and small pieces in your spice mill, or pound the daylights out of them after putting them into a small bag. Mortar and pestle works too.