There’s more to the auto transmission system than just the gearbox.
Why don’t you post up what problems you are having with your gearbox and what damage has been found as there are other parts that can cause problems with the auto box that can be a straightforward fix.
Possible faults include kick down cable becoming stiff, coolant in the gearbox from the radiator, valve block coils etc. It’s very easy for mechanics to condemn whole parts when smaller items may be the cause. I’m not saying this IS the case here but there’s a wealth of knowledge on these vehicles on here including the auto boxes which are known to be pretty much bomb proof.
How long did you leave it without changing the fluid?
I think finding an engine and gearbox or a gearbox and swap it for less than your $2500 is not going to happen, but it could be a case that the box can be fixed depending on what has happened to it. Alternately sourcing a good used box may be possible.