If you can find national brands with franchisees, you can often get the local or regional operators to piggyback the national buy to gain additional frequency at far less the cost they would have to do so on their own.
Look for co-op opportunities with a store. McDonald's will spend some money in small markets (speak with the owner if he or she is in town). Also your beer and soft drink distributor (once again, If locally managed or owned) will have some funds.
I have a co-worker who gets substantial agency dollars for one of the leading fast food franchises. Over the past month, he approached the local franchisee. Last week the franchisee signed for another 10-grand.