Hi clouds,
You might find a combination of calcium and magnesium helps.
I know that Bonnie Grimaldi, who has done some research into magnesium supplementation and tics for example, suggests ratio of 2:1 calcium to magnesium so that they will be absorbed correctly. Might be worth asking someone else about that.

I take a powder form. It's got some other minerals in it as well. It helps me sometimes with my RLS although I don't actually have muscle cramps. A cheaper way of seeing if it helps you is to buy the epsom salts and use that in a bath.

There are some helpful posts in the Vitamins, Nutrients, Herbs, Supplements Forum about magnesium. http://neurotalk.psychcentral.com/showthread.php?t=1138

RLS doesn't feel the same as leg cramps. It's more of a gnawing edgy feeling inside with a need to move. Hard to explain.