The best places to get calcium for your chickens' egg laying needs are eggshells from your local McDonalds and/or any other large breakfast restaurant. Just tell the the manager why you need them and ask him/her to save you the eggshells from that morning/week and promise to pick them up that day or daily right after they stop serving breakfast. Take them home, stick them in the oven until they're very hot, take them out, put them in a sack and pound them or roll them with a rolling pin. My five hens MUCH prefer eggshells to oyster shells, and I only have to get them from a restaurant once or twice a year.