It's a tough one. You might be able to go through a modeling agency but they would likely charge you heavily. It seems the best models are the entry level ones trying to build a portfolio for themselves. You end up helping each other out. They help your photography and in return they get some nice photos for their modeling portfolio. So I guess if it were me I'd target models who are trying to make it, that maybe don't have a lot of cash for portfolio work, and would jump at the chance for you to photograph them. Just stress that it will be tasteful of course.