My suggestion is to keep 6 "benjamins" tucked away in the deepest part of your wallet. Where to look? Smaller gun shows and pay attention to whoever wanders in with a single gun.
Do be willing to travel 100 miles to shows. Hit garage sales, house and farm auctions. Flea markets. i have had good luck at gatherings of "coon hunters" or hunting clubs.
Its a matter of being in the right place at the right time, doesn't hurt to have a bore light with you or a Harbor freight free flash light,