Pickupman, Classic Heartbeat or GMCPauls.
I just bought a pair of cab corners from GMCPauls. This is kind of embarassing as it was just earlier this month, but I don't remember for sure, but pretty sure they were Tri-Plus. Regardless, they were good, solid, thick-metal panels, and the guy who welded them in for me said they fit great.
The price is reasonable and they ship fast.
http://www.gmcpauls.com/Patch_Panels.htm
I had rockers that I bought from Classic Parts. One had a GM sticker on it, but it was the last one he had. The metal on the other was noticeably thinner, but still a quality panel IMO.
Good luck.