Check www.corral.net and www.stangnet.com in classifieds. If you find one that is in decent shape and just needs I rebuild I rebuild them....for alot cheap than a shop..
All depends on what it needs. But I can do a basic rebuild with all new syncros, bearings, steel bearing retainer for about $450 US. The parts alone are about $200.00 US or more. Finding a good core is usually the problem.