If you have a Lowes or home depot or such near you, you can get play sand or even bags of top soil. We have nothing close to sand on this farm. The chickens use the rich garden soil. They look kinda icky when they first stand up after bathing, but they shake it off.
Oh, I almost forgot something important! If you want to make SURE your chickens will go for your dusting area, just plant some nice perennials nearby and keep shooing the "girls" away from the area so they don't dig up all the plants.........that should do it!