I did go to a pet store but the only grit they had on hand had oyster shell in it so I didn't get it. Glad to see that @Poultry parent has had luck with the low-tech dirt method.
My broody hen is offering the chicks all kinds of treats and bugs. I had forgotten about chick grit and none of my local feed stores carry it. I ordered some from Amazon but it won't arrive until Monday. In the meantime I am hoping that they are getting what they need from the dirt and sand on...