As everyone else has said, you are going to have the 4L60E or 4L80E, the E being the important part here. That means big $$. However, there should be a large variety of tranny shops in your area that sell rebuilt transmissions. One place that ships nationally for not too unreasonable prices is Phoenix Transmissions (
http://www.phoenixhardparts.com/). My uncle and I ordered new transmissions from them. Their service seems to be a bit slow, but it's probably because they have a pretty good backlog going on. My uncle just got his tranny in when he ordered it before Christmas, and I still haven't recieved mine. You might want to give them a call. Otherwise, depending on what happened, you may just want to get your existing one rebuilt.