Need new computer, have some questions.

Hey everyone, my fiance owns a 1996 Firebird 3.8. For the past year and half her car has been getting exponentially worse. It started out with her car not starting every once in a few weeks. Now it's gotten to the point where it doesn't want to start but a few times a day. I'm taking automotive classes at my local college and after all the diagnostics we came to the conclusion she had a faulty ECM. The computer is malfunctioning and screwing up the car. My question? Where can I find a new (or used) ECM for her car? Will an ECM out of a 97-02 V6 Firebird work? How about the ECM out of a 96-02 V6 Camaro? I know I'll have to go have it reprogrammed, I'm just wondering if they are all interchangable. Thanks in advance!