I bought some neat horse fabric (not hunter/jumpers, but herd running, silhouettes etc) years ago from Wal-Mart, when they still had fabric. These days, you might try Joann's Fabrics--I seem to recall seeing some there. I can't check our local store because they're moving and won't be open for another month or two (arrgh...). They do have a website.
How much do you need? I'd have to dig through my stash to see how much I have, but I could probably send you some (free) to get you started.