Chickens have survived for centuries without much help from humans. You can buy some bags of "play sand" from Home Depot if you want to, and put it inside their run. They will scatter it, bath in it and ingest it. They will get their grit the natural way, like chickens have always done! Grit and calcium are two different things. Grit helps with digestion. Calcium prevents fragil and thin egg shells. With modern feeds, additional calcium is usually not needed, unless the eggs are too brittle. In that case you can save your egg shells and run them through a kitchen blender, before adding a little to your chicken feed.