As multiple people have pointed out throughout the day, the Derpy toy on Hasbro Toy Shop keeps popping up with stock then immediately selling out. A couple of people have confirmed that they were able to buy one during these very short ~2-5 minute bursts, so you may be able to pick one up after all!
Two intrepid Derpy customers have sent us emails from Hasbro Toy Shop customer care, saying the extra Derpy's are people ordering multiple toys from the same address. They are slowly culling out all the cheaters and selling the remaining in batches.
Anyway, you can find the page here! Good luck!