As above, look in gun shops and especially pawn shops.
You can also buy them on internet auction sites like Gunbroker.com. but most will need to be shipped to an FFL Dealer to be legal.
And you can do yard sales and antique shows.
Depending on what you want the outcome to be, you shouldn't rework an old Damascus barreled shotgun because these are no longer safe to shoot.
If you want a nice looking wall hanger that's okay.
If you want a modern gun that "might be safe" to fire, look for a gun made after the early 1920's but be aware that even modern guns can be unsafe in many cases.
As example a brother of mine had a 1930's Remington double barrel blow out a barrel on him.
As for guns like pump or auto shotguns, these are usually safe but you have to know what to look for to get a safe gun.
Many guns needing to be restored are in very bad unsafe condition, or may have been damaged by home gunsmithing, which is why people often dump them.
If you want to come out with a safe to shoot gun, make sure it can be made safe before wasting money on it.