Where and how to replace transmission pump seal? on 1990 GMC Rally Wagon 2500

I actually have a 1984 GMC Rally STX, but I figure the vehicle and years are close enough. Where can I purchase a used/new pump seal for my Turbo 700R4 transmission?

Asked by for the 1990 GMC Rally Wagon 2500
its more than just the seal. if it is leaking, you will need to replace the seal, the bushing inside the front pump, which made the seal leak, and possibily the converter if it is scored. USED PARTS WILL NOT WORK. you need to do it right.

man that sounds like a crapload of work