Sources for used guns include GunBroker.com, pawn shops, and all gun shops.
The best source for a used gun would be a reputable gun shop.
In my own experience, a well-maintained used gun gives the buyer the best value available for his money. Jean and I own very few guns that were purchased new.
I suggest establishing a friendly, personal relationship with a good local gun shop, and then doing the same with a good local gunsmith too.
Once a friendly relationship has been established with a shop, you could arrange to buy a used gun with return-for-credit privileges. The next step is to take it to the gunsmith for evaluation.
If the gunsmith advises against the purchase, take the gun back to the store and trade it for another possibility. And so on.