Since you are in the US (thanks for including your location in your signature, that helps) the grit you see at the feed store is what you want. It’s made of granite, is insoluble, and helps them grind their food in the gizzard. In the UK and other places, grit can also mean soluble oyster shell, talk about being divided by a similar language.
In the soil you describe they should be getting sufficient grit on their own. However it’s pretty inexpensive and it certainly doesn’t hurt. That comment on the bag Mymilliefleur showed “feed with coarse grains” does not mean to mix it with their food. It means they should have grit if you feed coarse grains but that also means practically any treat. Do not mix it with the food but offer it separately. You can put it in a separate container but why not just pour it on the ground where they like to scratch so they can have the fun of finding it.