I am sorry I can't help you with a purchase source, but it wasn't for want of trying about two years ago. I had looked pretty hard, and our jackscrew was made by a German company, but I could never find a US distributer.
For what it is worth, we had our thruster motors (both the big one and the jack screw motor) rebuilt at Broward Armature in Ft Lauderdale. The little motor gave them a LOT more trouble than the drive motor, mostly because of the fussy brush holders. Both eventually came back in excellent shape.
Hopefully they learned something on ours and can do yours quicker and easier!
Rock Sound Harbour, Eleuthera, Bahamas