I've had good luck on a few things on ebay. I won an original repair manual and original saddle for an early 70's Collegiate on there just a week or two ago. Because there are a lot of old Schwinns out on the roads, a fair amount of parts show up on ebay. I'm about to fix up an old Varsity for a friend as well, and I plan on getting some of the parts I need off ebay.
As for an actual store, I can't really help you there, but I also haven't really looked for one. As another Schwinn owner, I wouldn't mind finding one myself.