Basically everywhere anytime your driving I always keep my eyes open.
These old trucks are getting fewer by the day and less and less are out there for the taking. I couldn't find what I wanted for a decent price so I bought a couple to build what I wanted. Back in the 90s I used about 4 trucks to build one. Right now I will buy any that don't have a stupid markup. Which is hard to find since they have become more and more popular every year.
My point is if you have cash they are out there if not be resourceful and piece one together.
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.