I don't know if anyone has mentioned this option to you or not, (your post is a few months old), but what I do to provide calcium to my chickens is save their shells after eating an egg or egg. After you get a few saved, put them in the oven to bake on a flat pan for a few minutes at 350 degrees. This causes the shells to get brittle. Allow them to cool, then crush with a potato masher and feed the bits of shells back to the hens. They will gobble up the shells and get natural calcium from their own egg shells.