Just a chunk of ground/grass from outside.
Peat moss is a great dust bath, you can get it from most hardware/garden shops.
I use puppy pads for the first few weeks in my brooder. You can get a pie pan for a bathing area.
I Honestly wouldn't worry about germs or diseases. You can put a bit of sod in there for them to pick at. I wouldn't use sand as a brooder medium, they're likely to gorge on it.
It also gets everywhere.