I just did this upgrade for my car. I used
www.car-part.com and searched for an 88GT compressor. The results list should verify the donor car. It wound up being a little more than I expected, price-wise ($75 shipped) for a used part, but I got lucky when it arrived and turned out to be a recently remanufactured unit. I had my mechanic bench test it (it wound up needing a shim in the clutch, my mechanic and I are a little too thorough sometimes). My 87 GT now blows a chilly 40° with R134a. The nice thing about that website is that the yards that sell on there typically offer a warranty.