I might try to get a generic jingle tune down, then write specific lyrics for 5-10 (large?) local busineses. Then approach each of them until you get a buyer. Tell them you are shopping it around too so they know. I'd think they would want a finished product to evaluate your initial worth. Then use the first sale as a stepping stone to future jingles (either with same or different businesses.) Get friendly with your local radio station as they probably hire jingle writers.