Try the Kingsnake.com and Faunaclassifieds.com forums. Many of the people there are either breeders or knowledgeable and experienced keepers. If you can't find one near you (which you probably can), they're usually willing to ship the snakes.

The FC forums also have sponsors listed, most of which are breeders, if you really want a strong reputation behind the source. My advice is to make sure you get photos of the individual animal you're interested in. If they won't give you one, buy from someone else. Also make sure you ask lots of questions on both the animal and the seller's policies and guarantees.

A trustworthy seller is someone who isn't asking outrageous prices, offers individual photos, and answers all questions quickly and in a straight-forward manner. If you're not sure, feel free to ask...well, just about anyone on these forums.

Happy shopping and good luck with your new snake!