You ask every business that you are a customer of. Dentist office, bank, feed store, vet office, farrier, mechanic, restaraunt, gas station, bowling alley, etc.
Usually the horse show organizers will have a form that details what it costs to sponsor a class and it will ask for all of the pertinent information from the place of business so that it can be announced or displayed at the horse show.
Perfect Jin N Scotch - 2004 APHA Palomino Overo Gelding - Western Pleasure and Showmanship
Hanks Rainy Sky - 1998 Black and White Tobiano Paint Gelding - relaxing rides and blazing trails