I'm a cheap bas...er, I mean, I'm thrifty in my hobby. I use crushed oyster shell and aragonite to provide calcium for my crays, crabs, clams, and snails. I'll also toss in the remaining slivers of the calcium and sulfa blocks I keep in my matamata and FRT tanks when I replace the turtle's blocks.
Crushed oyster shell is sold in 50 and 100lb bags. But, you can find small containers of crushed oyster shell in the bird department of your lfs. It's the grit sold for bird's crops to break up their food. Aragonite is calcium carbonate and lasts around 2 years before you'd have to replace it/add to it.