Try posting on some Facebook Horse groups in your area. There may even be some groups specific to your discipline!
That's so exciting, and such an amazing idea! My biggest recommendation is to start small and gather together some of your barn friends you are close with and pitch the idea :) Ask three or four people to show with you and then expand it to 5-6! Once you guys get close you can prep for the shows together, wear matching shirts and Zocks (gotta have the look right)! Haha!