How do radio stations that don't have adds make money?

It could be either college radio or public radio. College radio uses student fees, donations and volunteers. Public radio uses donations, and volunteers. The money they get only goes to operating the station. Radio stations like this could have as few as one or two paid employees. There are lots of regulations that the FCC has on stations regarding what they can or cannot advertise, and still operate as a nonprofit organization. If you live in a rural area, sometimes you'll also find farmer sponsored radio stations that are cheaper to operate because they do not reach as large an audience.