Expect to pay $900-$1,200 at a tranny shop to rebuild one. Several are here on "classieds" for around $150. Salvage yards would be $100-$200 as well. You can get a used one, change the imput shaft bearing, front and rear seals, and it will run for another 200,000 miles.
Do a search for ATK, they rebuild blocks and transmissions. It was around $900+ for a rebuilt tranny with a warranty, not including core charge. You can also get ATK from stores like Advanced Auto sometimes.
I got a 5-speed transmission from a 77Z from ASAP motors in Houston for $450.00 last November. I believe they gave me a 1 month warranty. It works great. My mechanic said it was one of the best used transmissions he'd seen. They are at ASAP MOTORS.com. Good Luck.