If there's sand in their yard, I don't see why they would need you to buy any. (For reference, I don't buy chick grit at all. I use sand as bedding in the chick brooder, which I collect from a natural deposit near where we live. Have successfully raised 3 batches of chicks this way.) But if the chicken yard has pebbles too big for them to swallow, well, bring in some sand You can buy it at the hardware store.