For short term use, a pile of dirt will work.
For long term, pretty much any container that is big enough, low enough, and leaky enough to drain (if it is outside) will work.
I used an aluminum tub about 30" across and 12-18" deep because it was laying around not doing anything else. It worked reasonbly well for a few hens. It's not ideal because they would rather bath together.
If I didn't see something suitable laying around, I'd probably use 2x8s (edit: 2 by anythings) from the warped wood pile at the lumber yard to make an open-bottomed box. Or wood from pallets. Or cement blocks. Or maybe a worn-out tractor tire.