Sources....
Check Craigslist, E-bay, etc. Look under odd named stuff such as "Chevy step-side", "Chevy short wide" or just "Chevy pickup". Also try "farm truck" "farm pickup", "farm hauler"....that will get some less expensive results rather than Chevy C-10. Found ours under "pick-up Chevy". Had the best frame and trailing arms ---I'd looked at a bunch of trucks within 100 miles of Dallas for almost a year. Got the sucker (a 1964 short bed step side) knowing I'd have to replace 1 rocker panel for sure & two cab corners. Also enlist your friends to BOLO (be on the look out-give them a picture & a magnet to stick it on their dash) for you. Go to parts swap meets. Ask any local truck club members if they know of any.
Good luck with your search.