I'm not sure about it being a calcium thing, but would agree with the whole magnesium/potassium theory. We eat almost no salt at all, and both of us (the Man and I) would get cramps, usually in the legs, and adding a bit of even just normal sea salt (I think) to our food seemed to help. Except that we don't like adding much.. Now that I am pregnant and getting those pesky cramps in my calves again, I have tried to up the salt again, bit more water and walking to help the circulation. I think it works.
There are several different potassium varieties available from naturopaths (as I used to take them about seven years ago when I was doing heaps of physical work and going to the gym), as well as the different brands. I guess the best thing to do would be to talk to a naturopath or two, and see what they recommend. Aside from price, I haven't found much difference between brands. (ie, Blackmores folate tablets, up near $20 - 30 for only 30 tablets or so I think, and pharmacy brand less than $4 for 100!)