I'm sure someone else will have an exact answer for you, but I think it was a 4T45E but there are differences in transmissions - same with engines. They are designed to mount differently. The is a metal plate on the transmission that tells the specific one. Can't you find one from a Grand Am - would think those would be more common.
If all else fails and you're removing your transmission anyway, then just plain old look at it and the one you want to put into it.