Some people are lucky enough to have a mill or feed store that carries flax for horses, but if you're like me and don't the best place I've found to get flax is from Horse Tech. Horse Tech produces a product called Nutra-Flax which is a stabilized ground flax seed with added calcium. I've been using the Nutra-Flax for well over a year on my horses and love the difference I see in their coats and in their hooves. I've also seen an improvement in several horses with arthritis.
All of the horses I have fed it to enjoy eating it. Even the really picky eaters gobble it up without a problem.
I also love the convenience of it since it is stabilized I do not have to worry about it going bad, grinding it up, or making sure it stays refrigerated.
Since flax is naturally higher in phosphorus than calcium, Horse Tech has added calcium carbonate to help bring it into balance.
One other thing I like is that in ordering the product off of their website, shipping costs have already been calculated into the price so the price you see is what you pay.