It's not the trannies per say. True, the syncros are rather weak, many lay blame on the gears being over built as the reason for syncro failure. If you do a search you can find many threads on gear lightning which many claim to help the situation. I just learned to double clutch and rev match...Of coarse, depending on your power goal, those pesky donuts can be trouble but I think they are ok up to 200hp or so. Someone with more knowledge then me will chime in shortly...
The supplier list is a great place to find parts...
THE LIST