The world is full of people who want to be models, and I guess people who want to be photographers as well. You can easily get people to model in exchange for prints or digital files for a portfolio. I have a rule with myself that I always pay models for jobs that I might use commercially, and it's usually $20.00 to $50.00 per shoot.
There are a lot of true professional models out there, and they're usually represented by an agency. They run in the $100.00 to $200.00 per hour range, and will want licensing fees for usages outside of what's agreed upon in the original contract. Unless you've got a client paying the way, or you think there's money in stock sales for the shoot, this is not the way to go.
I once took a classified ad out looking for models, and I must have got several hundred calls. Now I use ModelMayhem.com to look up models (and they'll tell you how much they want right on the listing). Craigslist.org is another good place to place ads for modeling, or to look for people who want to model. You might also try the local modeling school, as there's alway people wanting free portfolio prints.
David Buzzard's Technical Blog