Yeah, kingsnake is a good way to go. And you can use the faunaclassifieds BoI to check if the seller has any weird history of selling sick animals or shipping improperly.
Craigslist is a good way to find snakes within driving distance that need new homes. It's the equivalent of getting a pound puppy. But there is more risk of the person not actually sexing the snake correctly.
Oh, don't forget to ask what the snake eats! Adults and subadults are usually more set in their eating habits than hatchlings. If the snake only takes live ASFs, and you only feed f/t mice, you may want to find a different one.