There's nothing really on the impeller that can go bad on those things. If the impeller won't turn, it's most likely one of two things. First thing... does the magnet on the impeller spin freely on the shaft? If not, then pull off one of those rubber bushing thingies on the end of the shaft and pull the magnet/impeller unit off the shaft. If there are deposits on the shaft, soak it in vinegar for a couple hours and scrub it with some steel wool. You want the magnet/impeller unit to spin freely on the shaft. Whenever mine seizes up, it's due to gunk build up on the impeller shaft and a good cleaning makes it work again like new.
If that doesn't work, then the motor part is most likely dead. If that's the case, just buy a new one.
Parts can be found on just about any online shop. Here's the section at drfostersmith...
Aquarium Powerheads: Maxi-Jet Impellers
Pretty sure ThatPetPlace also carries them. Seems like they carry every spare part ever manufactured for aquariums!