onecraigs.com is a somewhat tedious but good way to span the country looking for trucks (or other things for that matter). You can just type in "1971 C10" "1972 C10" et al. Autotempest.com is also good but takes more "configuration" (entering search terms to narrow down the search to year make, model, geography) and the searches go slowly.
If your budget is a little higher, many dealers advertise on
www.cars-on-line.com but you are less likely to find your "barn-find" "patina" trucks on there as you are restored examples.