Your best bet is probably Richard and Gayle Ray of Silver Meadows in Anthony, FL. They are just north of Ocala. They train driving horses that are competitive on the National and World level.
Don't know how far they are from you, Pikeville Tennessee. These folks, David and Dora McBay, do a great job of training driving horses and the folks that own them.
In June we went to the Area III show in Perry, GA. There were several people there from the Middle Georgia Miniature Horse Driving Club. This is their webpage address: http://www.mgmhdc.com/