So try magnesium citrate. You can get it from a pharmacy, in powder or liquid form. I have not used the liquid, so I can't tell you what dose, but if you do the powder, use it the same as the espom salts...4 tablespoons in 3 cups of water.
Go for the liquid form, however. I believe it has a better taste. The powder tastes kinda like the tang of candy--remember pixie sticks? The tang, bitter-sour, without the sweet.