Go to gun shows. Most sellers there are pretty savvy so don’t expect a steal, but you can get quality used firearms cheaper than new.
Also look in pawn shops. Be cautious; some of them are junk. But you occasionally come across a great deal. And the cops keep a pretty close eye on guns offered at pawnbrokers so you shouldn’t have to worry about getting a hot Saturday Night Special.
There are also publications like Shotgun News that have classified ads for used firearms.
You can score some great weapons at pawn shops, just do your homework and be prepared to walk on a bad price. Don’t be rude just thank them and walk off. Come back in a few weeks and make the same offer on the weapon if it is still on the shelf, they might just take you up on it and they love cash. Fail to do your homework and you can get skinned alive.