There are different kinds of calcium supplements/oyster shell products out there... I use the old standby from Manna Pro that looks like little white rocks, that consists of shell particles & coral calcium. But they really prefer the real oyster shells, which I've been getting from Walmart: https://www.walmart.com/ip/Vibrant-Life-Poultry-Oyster-Shells-5-lbs/796742456 . A bit expensive, but you may be able to find a local source that's more reasonable.