The EAA Warehouse seems to have the best selection. I usually buy my shirts on Monday morning as all of the common sizes (M - XL) tend to sell out early in the week and are pretty much picked over by Wednesday.

If you missed out a shirt that you like, watch the online EAA store. After AirVenture they usually pull all remaining inventory and list online, which often includes shirt sizes that may have been sold out at the show.

I usually buy a 3x shirt every year for a relative who can't physically make the show due to a disability.