Where can I find an air conditioning unit for my 1994 Ford Mustang GT??
Fordman has it right. The junkyard is a good place to get what you need. I would only add that you MUST get the parts from a sealed system that has freon in it. If you get the parts from a system that has been open, your asking for more trouble than it's worth to even do in the first place. In fact, if the evaporator is still in the dash, and has been open all this time, it will will need to be flushed very thoroughly, or, better yet, replaced.
And good luck, if need any more info just yell!