magnesium

Yesterday I had cramping in my calves while lifting...and I wasnt lifting legs. Anyway, I think that was a symtom of magnesium deficiency. correct? What foods can I eat that has a lot of magnesium? Also I have a couple sample packets of emergen c. It does not contain a lot of magnesium but I remember you saying it was good for electrolytes. But there is 5 g of sugar. Should I take it anyway?