Google "magnesium chloride" and see it there is a health food store near you that has it. Some use it as a suppliment. Other than that, I have seen it in non food grade form (ice melter), which I would not use.
You might want to look at more industrial outlets to see if you can get it. There is a guy I know that orders it from a chimical supply house, but he has an account with them for his profession.