Mag Chlor is a mined product. While we do mine some in the US, it is snapped up by those close to the mines, and little escapes from that area.
So most of we see is imported and it is expensive and not easily found in some areas.
When we see it in stacks at the big box stores it may say it on the label, but read carefully, it almost always has other ingredients, such as Ca Chloride, and plain old salt, The latter two are not environmentally kind. Either to your land or your horse’s feet and legs.
So read before you buy.
I have ridden on Mag chlor for over 20 indoor years. Norespiratory or foot problems. AND it seems to last. BO skipped last winter, and this summer it was still keeping dust down. I believe BO uses normally about 1400#'s /year for a 200x 70 arena… That’s 10 bags of flakes , top dressed, and dragged in.