Definitly check out horse stables in your area. The place where I board our horses has tons of it! Usually it's load your own, unless they happen to be spreading on the hay fields at the time. If you time it right, they'll load it and you'll get the good stuff thats at the bottom of the pile.
Cow manure is better though, a cow's digestive system eats up seeds that are in the grass / hay. A horse only has one stomach so the seeds don't get broken down. If there is a dairy farm or feed lot near you try them.
Bud