Another source for models is that subway ride on the way to and from work. I've been doing this since High School and have learned that by the end of the week you can catch people napping long enough to do a quick drawing. I also began to tint the paper with a warm color in order to aid in getting the image down quickly. I would use the tint as the middle value and use color pencils to work on the darks and the light. Some times I'm a little timid about starting the drawing because I really don't want to have a conversation about what I'm doing, or I don't want the person I'm drawing to suddenly have an attitude about someone starring at them on the train. But when I work pass that I usually get on a roll that will keep me drawing in my sketchbook for months.