You can try contacting Steeda, but I'm pretty sure they don't make it anymore. The best you can do would be to convince them to make a new one if they still have existing molds. I imagine that would cost a fortune.
Cervini makes a kit that is a little different, but still pretty close (might be closer to the old Saleen kits). And they make good quality stuff. In fact, a new kit from Cervini will probably fit better than an old Steeda kit. There just isn't a lot out there for the old Fox body Mustangs anymore.