It can be pretty expensive to have one rebuilt. I believe it's Hanlon Motorsports that sells the video on how to do it, and you can pick up a rebuild kit from either Hanlon or D&D. GreenMustangGt rebuilt his, so he could probably tell you what all it entails.
It depends on how broken it is if you want to rebuild it yourself...sounds alot worse than just replacing synchros. Sounds like the counter shaft or 3rd gear itself. Parts can get pricey if you dont know exactly what you need.