Friends, family, approach people in the street (if you're comfortable with doing this), small advert in the local paper, contact local businesses to see if any of them require head shots of employees for web sites, who's who boards etc., local art clubs etc. sometimes have a register of models and I've found these tend to be less expensive than web based model sites.
A photographer I knew contacted the local hospital and explained that as a project he wanted to take portraits of the staff within their surroundings. He ended up with literally hundreds of pictures ranging from adminstrators, surgeons, porters, cleaners, voluntary staff etc. The hospital ended up buying many of the pictures for their walls and they were showcased in some magazines.