I've heard good things about redmond rock salt. Going to pull up FeedXL to see what they say.
Calcium, zinc, iodine, chlorine, phosphorous, iron, magnesium, copper, manganese, sodium, and potassium.
NOTICE NO SELENIUM, so make sure to supplement that if it is not in your horse's diet.
They have it at smartpak (rock and selenium supplements). A 7lb rock or in smartpaks. Thinking of getting a rock for Sky for awhile now. And selenium crumbles as well.
Hope that helped! :)