older chevy pickup parts

need to get a 1977 4wd 1/2 ton pickup to pass inspection. having a difficult time finding the backing plates that the rear brakes are mounted to. any sources of parts would be appreciated, all our local junk yards don't have any older stuff probably because it rusted away like the truck i'm fixiing. thanks

Look at picking up one of the LMC truck parts catalogs if they have one for your truck. Go to their website and order one (the magazine is free so they can get more orders). The catalog will be nearly every part for your truck, and a few aftermarkets and I believe it will have the backing plates you need.
-Jut-