Well...I'd have recommended eBay, but once we got good again....our gear got expensive again. Like, it happened so close together it is obvious. Used to be a great place to fine NWOT etc type vintage stuff. And maybe I am not supporting the Bill shipping there, but most of the time a sale did support an actual Bills fan.
I've gotten script hats ("the Griswold" as they are now known) from there many times, weird one off Bills event hats (QB camps, anniversary events), FOUR Bills leather jackets (Starter, natch), all VERY, very cheap. But it appears dem day is over.