There was one that was released at the beginning of the 100 Years Celebration and that is still available. There were four more released during the year, one for each park's parade, and only the most recent one (for the MGM parade) is still available. There is a AP Logo one that is not a Limited Edition so I imagine that will be available for a long time. There was one released for I Conquered the World with Tinkerbell and there may be a few of her left around. There was one for the Festival of Masters which is now being sold on the racks for anyone to buy (Moulin Mickey), and there was one for the Spectacle of Lights that may still be available at MGM Studios only.
One spot where I seem to have pretty good luck getting the pins is at AK - Island Mercantile. The pins are at the cash register which is closest to the park entrance/exit. You do have to ask for them.