Dust bathing is part of their grooming process. It could be a pile of sand in the run. some people will put sand in a container the shape of a kitty litter box.
My whole outdoor run floor is sand, which they love to take dust baths in. But, above everything else, they love taking dust baths in the dirt outside the run, under the bushes.
We have a run that is the earth, some sand, some rocks some leaves. We had a little truck bed cap that I wanted to keep in there so that they would have a place to get out of the weather and hang out if they wanted to. They have decided to turn that into a super huge communal dust bath. They all go in there, bury themselves in broken down leaf matter and dirt, roll around, kick it on each other and just plain old hang out in there. The first time I saw one of our new Leghorns come out I thought she had been "rolled" by one of the other chickens. She was a mess. They Love It!
I don't know if anyone else uses this for dust bathing, but I put 4 or 5 bags of mulch in my run. They go NUTS with it. They are content to scratch in that all day long. I also have two old tires filled up with soil for them to dig/dust bathe in. I tried sand, and they didn't use it to dust bathe.. Picky chickens.. Of course they like the combo of dirt/mulch the best.. Where my flowers are
My chickens LOVE ash! They will go to our burn pit and the white ones come out black! Uggghhh. I don't know if they NEED it, but if my chickens needed a golden roost... they would have it!