The best place that we've found is eBay or at the Disney Store on Clearance. These are both "hit and miss" approaches.
There used to be a pretty good site, but it's not there anymore. I'll do some digging.
There are three auctions going up in 15 hours that are currently more than $1.00 a pin. They all have lots of 50 pins. It is still early and a lot can happen in 15 hours.