I've been using calcium chips, found at a pet feed store. It's crushed limestone, about 1/2 the size of pea gravel, and used as bird food supplement to strengthen the egg shells. You might want to see if there are any poultry feed stores outside of NYC. It came in a 50 pound bag & was dirt cheap. It should last a couple hundred 4" pots. If not that, maybe a local pet store that specializes in breeding exotic birds might have something similar.