Hi all, new on the sight, from far away - Cape Town, South Africa. Have recently bought a '76 CJ7. Had leak on the transmission, but that not the problem. The 3rd cogg is almost gone, and looking for replacement.
If it is slipping, just have it rebuilt. Mine started to slip in third at about 150k miles and that's what I did. If no one there can do it, order a kit and do it yourself. I bet it would be difficult to find a replacement for the AMC TH400 where you are.
Best to rebuild it now. A bench rebuild job (meaning you pull it and reinstall it yourself) isn't usually too outrageous around here but not sure what's available in your neck of the woods. But like Will said, you can order a kit and a rebuild manual and may be able to do it yourself. I would have done mine but I found a great deal on a bench job w/warranty that saved me a ton of time. Good luck,