Others will chime in and tell you where to get it from, probably LMC. But personally I would rebuild it myself, it's a free fix if you spend a few hours doing it. I haven't rebuilt my rear window motor but I have torn the side window motors apart. Basically you drill out the rivets, clean up the dried up grease inside, clean the contacts on the motor, flip the motor brushes around (to give you new brushes), put dielectric grease on all the electrical contacts, regrease the gears, and reassemble the case with nuts/bolts.
Are you sure it's the motor that's bad and not something else like the wiring going to the motor or dirty window tracks? Have you tested the motor by hooking it up to a 12V supply?