I recently bought a 50# bag of calcium for my girls. I have always used oyster shells. My feed store got this in and it said on the bag for poultry so I'm trying it as they were out of oyster shells. Not sure what it is made up of and I took it out of the bag and put it in buckets with lids.
I always grind up their shells so they barely resemble shells (so they don't get any egg eating ideas) and it works great. I leave it in a seperate container that they can freely eat from. If you can grow alfalfa, it helps them retain a lot more calcium.
If you are going to feed egg shells, bake them in the oven first then crumble them. Yogurt is a good scource of calcium. Homemade is so easy. If you make your own add some nonfat dry milk to the milk before the culture is added. Boosts the calcium