most calcium supplements are calcium carbonate.

For people with kidney issues, or a history of stones, then calcium citrate is used. (CitraCal).

Anyone of course can use the citrate form. It is slightly better absorbed.

For best results Vit D should taken also, and I'd suggest the new guidelines, for dosing.

There are foods now enriched with calcium. I use a Lactaid milk that has 50% of the RDA in it. There are also calcium enriched orange juices.

There are limits to how much can be absorbed at a dose, so breaking up the total amount in divided doses is best, rather than taking it all at once.

I prefer Maalox rapid dissolve multisymptom--it has simethicone for gas, and works well for me. I don't take it every day since I eat calcium rich foods.
A cup of yogurt has about 400mg calcium in it. My husband has a yogurt every day with breakfast.
Vegetables like broccoli have about 100mg /serving.
Swiss cheese 275mg
Salmon 250mg

Calcium interferes with absorption of some drugs:
Cipro and other fluroquinolones
Tetracycline family
phenytoin (anticonvulsant)

There are studies coming out now showing using calcium alone with no Vit D, is not efficient or worthwhile, so attention to Vit D intake is very important.