I've always bought my compressors off Ebay. I can usually find a pretty good price there. I never got one that was completely bad, but one i got only lasted 2 years -- the others are still OK. I got one for my red SE (my brother still uses it), two for my yellow t-top (including the two-year one), one for my silver Formula, and one for my newest white Fiero.
Edit: Keep in mind that the V6 Fiero and 86-88 4-cylinder Fieros don't use the same kind of AC compressor. Make sure you get the right kind. For the V6, it's the HR6 compressor.