Yeah, I've saw some that look REALLY similar to those on americanmuscle.com . Its a site for Mustangs, go to the complete tire and wheel packages and browse. There close, the offset and everything.
You should check out Boost Auto Parts. They make NBS Tow mirrors just for GMT-400"s , with the correct bases and wiring. I'm currently saving my lunch money every day for a pair. They look killer on our trucks!