Well, assuming you are indeed a collector, what parameters have you set? Saturday Night Specials cover a lot of territory, and some very interesting handguns. Very old guns would include the Sharps four barreled pistols of yore,along with the Colt cloverleaf revovlers. Add to this the French Apache type guns favored by prostitutes and pimps, and you need a cut-off point somewhere.
As to sources, pawn shops are certainly the very best place to find Saturday Night Specials, as well as gun shows. Older guns come under the relic and curio class and can be found in second hand shops and junk shops. Second hand shops going under the "almost Antique Shop" variety are often a treasure of old guns. Yard sales sometimes produce such guns, but not so often.
And, used furniture shops often get furniture with guns hidden away in drawers or chests. Look and ask.