Try putting an ad on Craigs List.
Checking out the ADS website for any members in the surrounding areas.
Put ads in the breed registries for drivers, mini assoc, AQHA, Paints, Morgans, Arabs, Saddlebreads, ect. Any horse that drives, also the Draft and Mule registries.
Start a facebook page. Check out prize lists from past shows in the areas that had driving. Post on the Carriage driving Classifieds, The carting Zone, Carriage driving on line, Draft horse sale and trade, CDE and pleasure driving, all on Facebook.