Try searching for radio stations that broadcast in that area. Visit their websites and see if they give information on how to contact their advertising department. (I would be willing to bet they will have that information readily available, since advertising is their primary source of revenue.)
Before you advertise on any given station, you'll want to get information on their audience demographics. You want to make sure their listeners include your target audience. There's no point in advertising acne cream on a radio station that caters to a 65+ year-old crowd, for instance.
If you cannot figure out who to contact at the radio stations, or you cannot find suitable radio stations on your own, or if you have little to no prior experience with media buys, you should consider hiring an advertising agency in the area who has experience with radio advertising.