I have had good luck with panels (lower door and lower fender) from Classic Parts
http://www.classicparts.com/That being said I do agree that using OEM sheet metal is the best way to go (if you can find good rust free parts that is).
Alot of this depends on which panels are being replaced. For rockers I wouldn't be to concerned.
For instance it is my understanding that the repro front fenders are miserable in the area of the grill. The factory dies provided sharp - crisp body lines, the aftermarket parts, not so much.
Hope that helps.
John
