The Community of Disc Golfers and About All Things Disc Golf
Getting discs from your local Discraft/Innova/Gateway sponsored player goes a long way too. Usually they'll sell for 30% cheaper than retail and there's no shipping costs. It's a win win situation.
The Discraft factory.
Some stores may have a few hundred discs but nowhere else has thousands and thousands and thousands. The lines are short and the prices are so good it almost as though they are free.