I keep an eye on local auctions for stuff like that. They don't really show up anywhere else around here - even the pawn shops don't want "mostly complete and partially broken" firearms.
Last year, I picked up 4 long guns - 2 shotguns and two .22s - plus a pellet rifle, from a place that does online and in-person bidding on bi-monthly auctions. I don't remember which price went with which gun, but the winning bids were $5, $7, $8, $11, and $13. Altogether, including some reloading tools, I was out the door for less than $75.
I immediately turned around and re-sold the pellet rifle and most of the reloading tools for $90, making the rifles and shotguns free.
One of the .22s turned out to be a complete basket case, and I've given up on it. (You can't always win.
)
But, the two shotguns were salvageable. One needed a spring and guide rod, running about $16. The other (which had been run over by a truck) cost me about $60 in shop supplies and parts - with most of the cost being for a magazine - and a lot of labor to repair the stock.
The other .22 turned out to be one of the best investments I've ever made. I think I paid $8 for the rifle, minus the bolt. For $35, I got bolt off Gunbroker, and it is now a family favorite ...in all of its beaten, battered, Bubba'd glory.