I have a 1999 pontiac 3.8 that needs a transmission. What other transmissionscare compatible as far as make and model?

Not quite sure but if you did an LS motor swap you could do a LS motor swap and transmission all together in one shot used from the junkyard if you want it's an upgrade. But I would imagine you can find your transmission used in the junkyard fairly cheap that would probably run the only way you can really tell what transmission your vehicle came with is by doing a VIN search It will tell you what your car came with.