I get NOW Calcium Carbonate powder from the local health food store.
If you have a local health food store, they should be able to order
it for you if it's not on the shelves already.
If you go to Now's website, they have a store locator, listing stores
that carry the brand
http://www.nowfoods.com/
You may want to consider the Magnesium oxide powder, too. Magnesium
(and zinc and vitamin D) will help the calcium get absorbed.