Where to purchase a A-500 / A-518

Sounds like your money is as tight as mine. I'm not interested in the performance upgrades and such , just the overdrive.
I saw a 93 Ram van with recently rebuilt trans, decent tires, etc.., drive it home, for 900 bucks on local Craigs List. There was my trans with a small junk stroage area to boot. I didn't have the coin at the time.