The precise form of calcium doesn’t matter that much. There are a lot of internet articles touting one form above the other, but for calcium most forms are ok. Calcium carbonate is widely used and is an adequate form of calcium supplementation.
This is however not the case for some other substances, such as magnesium. We prefer magnesium malate, as we explain here:
It’s also important to combine calcium with vitamin D and vitamin K, as we explain here: