A broody hen introduces them to grit as soon as they come off the nest.
You can usually buy chick grit at the feed store, usually. That's just scrap from a granite quarry that has been sifted to get the right size. You can do like a broody hen and just put them on dirt. They will find their own grit. You can give them a coarse construction sand to use as grit. KPGoldstar's way works. Get some sand from a sandbar in a creek or river.
There is nothing magical about grit. Chickens don't have teeth to grind up food. They use small pieces of rock in their gizzard in place of teeth. They'll use any rocks they can find, including larger grains of sand.