Pawngal, Have you investigated an ad in one or more of the tractor, truck or trailer advertising papers that are seen in Iowa feed stores, auto parts stores, or farm implement dealers? Another possible idea would be one of the auction services that dot the Iowa landscape. Perhaps they would allow posters or something similar at an indoor auction for collectibles etc.? Of course they may be concerned with competition, but maybe want to place there ad or poster in your shop.
I also see ads for a lot of businesses, auctions, cars, etc in Caseys or other convience stores. They usually have a bulletin board near the rest room, or even a rack with the farm and collectable auctions.
Sort of appeal to the typical Iowa, southern Minnesota rural market !
Our local used car place, speaks very well of their business increase using ICAN, Iowa Cable Advertising Network. They like their ads being during sports events, especially during the winter. You know how Iowa is about local sports teams at all levels. No idea about your budget, and ICAN costs though.