Our local composting giant bases his entire operation on the adjacent horse racing track. So YES, there is! He adds other materials from places that would otherwise have to pay fines to dump their materials down the drain. Places like dairies, breweries, and soft drink bottlers are not allowed to dump into our sewer system because the content and quantity of their waste kills the specialized sewage treatment microbes at the sewage processing plant. The sewage dumping fine is $1,000 per day (retroactive), so those places pay the composting company significantly less to allow them do dump their truckloads of waste on the compost piles. This is something to keep in mind as your composting operation grows. Other sources for horse manure are your local veterinarians. I have an e-friend in Alabama who started a composting business on dung collected entirely from veterinarian businesses. Search the web for CaptainCompostAL.