Dreamhorse.com is really nice...you can search for really specific things like location, pedigree, color, basically anything you can imagine! It is a pretty popular site too, so you should see quite a few options. Also, if you are on facebook, join a couple of the local "horses for sale" groups, sometimes its nice to find something that is really close to where you live!
Some websites are localhorse.com, dreamhorse.com, equinenow.com, horseclicks.com, horsetopia.com, equispot.com, and craigslist occasionally has some good ads. I found all these when I was looking for a lease (: Good luck!