Calcium is a dangerous thing to supplement. It has a cardiovascular consequence. Safer options are San Peligrino or Gerolsteiner mineral water(or similar) and you can also get celtic makai deep sea salt which has 2% rda of calcium and magnesium per 1/4tsp. And don’t shoot for the actual daily rda. Just intuitively drink as much mineral water or intuitively use as much of the salt that tastes good. You aren’t consuming oxalates,phytic acid, and other PLANT antinutrients when you are fasting so don’t get too worried about calcium.