So, if you still want a Derpy and don't want to pay 5-6 times the retail price, apparently Hasbro keeps putting a few more up randomly. Apparently, they're catching people who ordered multiple for one address, and putting those back up. They seem to sell out almost immediately, but if you're lucky, maybe you'll catch their store right after they offer up a few more.
I really wish she weren't a brushable, but because it is Derpy, I would probably make an exception if I happened to catch her for sale in the store.