Any sort of a broad, flat-bottomed, shallow dish works fine, such as the large, glazed pans meant for use as plant pot bases to catch water seepage from drain holes.
A smooth surface has the very great advantage of allowing you to clean it much more easily than any dirt-catching rough surface. Birds can either sit on the rim to drink or hop into the shallow water to bathe. They often defecate as they bathe so it WILL need to be cleaned regularly, as well as (optimally) sluiced out daily and refilled with clean water.
Placing it on the ground (rather than on a pedestal as with traditional birdbath designs) will allow a variety of animals to have access to water, very important in areas with scarce surface water or in times of drought.