Wanted to say 69 but the earliest truck I can dig up at the moment is 10/69 thus a 70MY build. Late 69 Camaros, that extended production into November of 69, carried the label starting 08/69 IIRC.
As far as suppliers go try these too
ECS
www.escvin.com (commercial autobody supplier)
Corvette Central
www.corvettecentral.com