Grit is crushed granite usually, and you want to stick with that.....the crushed oyster shell is for laying hens, and that would be too much calcium for your chicks, which can actually harm their development.
Where do you buy crushed granite? My feed stores don't carry it so I have to go to the pet store to buy parakeet/parrot grit which I bet is way more expensive. I know I could probably get it on line but I bet the shipping is expensive. Is there a brand I could ask my feed store to order?
most feed stores carry regular old grit in ?50 lbs bags......that'll last you for a good long time. Free ranging chickens can usually pick up grit/tiny stones as they forage