Good luck, I just went through the whole ordeal trying to find backup lights, retainers, and a rear valence for my 68 and it was a pain. My best advise is to "google" "used corvette parts" and just start calling little and big corvette salvage yards. You can expect to pay $140 (what I paid) to to $220 and thats just for the lights. I did however find one guy that had everything for sale, valence, lights, lenses, etc... for about $160 but you can imagine how fast that went.
Good luck, its a lot of calls but its worth it when you have it in your hands.
Corvette Store
68 corvette backup light assemblt
