Quad 4X4 is a great place to get one. He has the supplies to rebuild it yourself and a good source for knowledge. Do yourself a favor and upgrade the pilot bearing in the flywheel, will have fewer problems in the long run.
Ditto's to what Cruiser_2 said. I have a Quad 4X4 NV4500 with the torque King main shaft and upgraded pilot bearing. A good set up.