hslager69's link has the desired part photo'd and listed right there from Franks for a very reasonable $125 (right side only). 8th row down, first part shown on the row. I sincerely hope Rich69Bird snatched it up, as these cannot be easy to locate regularly...not too many dismantlers would vertically cut a perfectly good used quarter panel to sell a chunk like that...presumably the car that supplied this one had a damaged or rusted rear quarter section and only the front part was usable. Thus no left side for sale...only the right.

The part shown is a used section. There will be no cheaper or easier alternative than purchasing that piece, assuming he needed the right side. Claimed rust free, and it'll certainly fit better than any repro would. Now he just needs to find a left side...assuming he needs both? Rich69Bird didn't specify which side he actually needs.

If left side is still needed, I'd recommend searching Camaro salvagers for this piece. The desired chunk will be identical if cut from a Camaro, and more were produced than F-birds.

The other consideration here might be coupe vs convertible. When getting into the inner rockers these models become different. We're not entirely sure what kind of car he's even fixing...