Garden vermiculite is SUPER expensive as Urbangrower said, and chickens will shake/kick/scratch whatever you use for their dirt bath EVERYWHERE which means you will need to replace it often.
I recently got busted for my urban chickens and had to temporarily move them to a friends house in the country but now they have about 1/4 acre run in a large area where they can pick their own dust bath places. They seem to favor spots with dry/sandy soil and all hang out together in a communal bath hole.
When I had them in a small run on my tiny city lot, I used a large,rubber horse feeder bucket for their dust bath and filled it with a combination of woodash, DE, pineshavings, and old dry potting mix- they loved it- but as I said, it needed refilling often since they fill up their feathers with it- then walk around and shake it all out.