1990 Chevy Suburban Transmission Problems!

okay, when my tranny got warmed up and built up pressure the fluid started pouring out of the vent tube in the housing....I was told this is a pump issue.
how do I change the pump and where can I find a cheap one?

Welcome to AF, this should have been posted in the Suburban forum below. Anyway, that is not a proper diagnosis, overheated fluid, blocked cooler lines worn internal components all can cause this, the pump is the front portion of the trans it requires removing and dissassembling the front of the trans if this was the problem, new pumps can be purchased through any major trans outlet, but you are getting ahead of yourself here, have it checked by a pro.