Because equestrain facilities is such a narrow niche, it may be difficult to find an agent who specializes in just this type of real estate. I own and rent our horse property. I found the best way to find property not already listed in the MLS, is to go to Equestrian Clubs, e.g., Saddle Clubs, equestrian training facilities etc.. Generally the leader of each of the organizations are substantially connected and can help you find what you seek or refer you to an agent who may be equine savvy.
Don't forget tto check the local paper traders. Flathead Valley's Beacon and Mountain Trader have advertisements for such properties here in Northwest Montana. Nickel Ads and The Fencepost are some Colorado/Utah paper traders. Most of your farm/ranch wholesale/retailers will have a paper stand for these type of local trader papers.