I've had good luck with Craig's list and One Model Place. The models on One Model Place can be more expensive whereas Craig's List seems to attract more art models who understand that not all photographers are built of cash. If you post to Craig's List be sure and include your website and a phone number (I also include what I'm able to pay). That will set you aside from many of the other photographers looking for models (and dates).
Finally, don't forget to consider asking friends and friends of friends to model. My best models have always been someone I knew and they hadn't modeled before. They understood my work and collaborated with me in a way that many paid models haven't.