Serum blood level low? Or you know your low? Are you getting muscle cramps at all? How do you know this--

Do you have labs to confirm that you are low in calcium?
What is your diet like?

Have this test done to get an accurate reading of your current level of calcium along with other essential minerals.

http://www.metametrix.com/Testing%20Services/Element%20Profiles/Erythrocyte/

Once you are armed with good data you can supplement in various ways, wholefoods being the best (juicing, etc). Organic of course. Some would say to use coral calcium in a good blend with magnesium.