OK I've been through this a couple of times with 4-H and FFA projects. First What is the charity or event? After this is decided then pick up every tack or farm cataloge you can lay your hands on. Write to all these supplyers explaining what you are doing. Ask them for a donation to an auction you are putting on to suport said event. Heres the catch line. Make sure you ask them to send free catalogs so everyone who participates will have access to the advertiseing of their wonderfull great products. I would set the auction up at least a month in advance of when I started mialing out leters to supplyers to give them time to ship the goodies to you. Good luck. Also if you can have hotdog roast or BBQ at the same time and advertise it as free but donations for your cause would be welcome you will get a god response. Good luck.
You could do a lope for hope. You get a bunch of fellow riders and get them to raise money for their ride, we do it around where I live. They earned alot of money. People come from all over as well. You could even see if a trail riding facility will let you do your ride at there place.