You might try down in the Swap Meet area, Ebay and Hemmings are also good places to look. You can also find them at car show/swap meets, too. Don't overlook a passenger car version of the transmission, they are essentially the same as the truck version, there's only a few differences, depending on year. Mainly, you may need to swap the side cover and shift arms from your existing transmission. Expect to pay about $100-150 for one, expect to at least put new gaskets and seals in it. If possible, se if the solenoid works, as they are not available new (good used ones can be found, though).
I installed one in my 3200, best thing I ever did for it. It'll cruise at highway speeds easily, but still have the takeoff power for in town driving. I like them setup more or less as the factory intended, with the overdrive working automatically. I did add a switch to cut power to the solenoid when it's locked out for in town driving.
