They have them at Once Upon A Toy in Downtown Disney and also at MouseGears in Epcot. They probably have them other places too but those are the two places I have seen them.
I know this isn't what you asked for but for others who read - In Once Upon A Toy they also have the same thing for My Little Pony on the other end of the store than the Mr. Potato Head figures. They don't have AS much Disney special parts as MPH but some fun stuff still the same. Also, at Once Upon a Toy they also had some other special parts that our POP Century gift shop didn't have. We bought the Dumbo that they sit on but they also had a yellow poncho like the ones they sell at the parks when it rains and a few other things that I just totally blanked on. Anyhow, the Dumbo is big enough for the big MPH to sit on and was only $5.00 - a price that we thought was great and our girls love pretending that MTH is at MK riding Dumbo - their favorite ride of course!!!