Snail food [speaking of calcium]

swarvegorilla:

A handful of shellgrit never goes astray
or ya can mix pure calc carbonate powder into their food.

The guy I got the snails and daphnia from gave me a handful of what he
called calcium carbonate, looks exactly like the stuff sold by pet stores
for removing nitrates (little white rocks). I've looked in stores and I
can only find two labeled sources of calcium... calcium sand (kinda
expensive) and turtle bones (cheap). What is my best, cheapest option for
calcium?
--
Mac Cool