Although I haven't fed BOSS, it is popular here for non-horse livestock even though it is expensive.
I use whole flaxseed and yes it can be hard to find.
If there is a good feed mill nearby they usually have the ability to order it(as well as just about anything else you could want) in for you, or call a larger feed distributer that caters to cattle, sheep, goats, llamas, etc... and not just horses.
My flaxseed comes in 50# bags from either a family owned feed and supply store or the mill very close to them where I get my organic kelp and poultry feeds. It is feed quaility flax which differs a bit from human grade flax and is far cheaper.
In addition to feeding the flaxseed whole, I also feed my animals a species specific vitamin/mineral mix and both my horse and my dairy goats wow and amaze people at how shiny and healthy they look with good hooves all around.