For electrolytes :
You can easily take potassium and sodium in the form of electrolyte drink I make. See the recipe here. I just take magnesium supplements and it is a must for the people on a low-carb diet. I have used both the supplements given below and I prefer both actually. Always look for chelated magnesium before buying any supplements, as it is easily absorbed by the body. Do not buy magnesium oxide at any cost . If you have to chose between the two (below), do it as per your budget.