You can also try Quantum if you strike out finding a rebuild-able unit. They usually have a rebuilt 3-4 OD box on hand and ready to go. IIRC, they go for ~$2500 before shipping. Getting an OD gearbox to rebuild will be in the neighborhood of a grand, then you have to pick through the box and OD. So going with Quantum or another similar shop might be a wash as far as total investment goes if you factory in the time to locate, ship, and rebuild an old gearbox.