I have had good luck with eBay, this site's classified ads and every once and awhile the local pawn shops.
A friend of mine has had great luck with estate sales.
For online purchases, take your time, scrutinize pictures and ask questions.
A lot of folks who sell off a gun are clueless about the gun. The family finds an old gun in a mans stuff after he passes away and they assume the dusty old fashioned gun with the scratches on the stock is likely of no value unless it is a Revolution style long rifle. Then they get to thinking it is a real deal...those folks make me sad for them. They think it is worth much more than it is. But it is not a war relic at all. It is grandpa's take on a build of a Lancaster he made 40 years ago. Hard telling them that they do have a valuable gun, but it is worth a fraction of what they thought.
Back on topic:
Use many sources, go into stores, go to estate sales, look online and ask around.