I’ve been involved with some of the local equine-assisted therapy organizations where I work. One holds a very successful Ride-A-Thon every year. Riders get sponsored by the mile and then have a fun trail ride on a certain day.
Also, it’s been my experience that horse owners love continuing education and will pay to attend owner seminars. Dog and cat owners generally do not. You might consider contacting some of the large feed companies that are trying to break into the equine market - Moormans and Cargill are two that come to mind. Perhaps they’d help sponsor a Horseman’s educational seminar. If your place is like Indiana, they’ll provide several hours of CE on equine nutrition. Throw in a breeder or a veterinarian and you’ve got a nice day of education you can charge $50 or bucks for.