I believe Napa sells a rebuild kit for the gear box, but I don't know about new or rebuilt ones, never asked.
I feel sure that rebuilt ones can be bought somewhere, but you probably don't want to pay for one. Your best bet is to find one that is in decent shape used.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-