Check out the collectors board, there is a comprehensive thread of good/bad internet pin sellers. Alot of good DIS members have contributed to keep us away from counterfeits.
Also, if you are planning a visit to the Character Warehouse outlet, they have pins for trading. There was a temporary store up here at he beginning of the summer, and I picked up several booster packs, for 6.99, which is less than $2 a pin.