Pololu Ball Caster
This ball caster kit includes a black ABS housing, a metal ball (chose the correct size above), two spacers (1/16" and 1/8" thick), and two #2 screw sets.
The total height of the ball caster, 0.83" or 0.53", can be increased to about 1" or 0.7" respectively using the included spacers. The two included #2 screws can be used to mount the ball caster to your chassis (the distance between the screw holes is 0.61" or 0.58").
Note that the ball casters are designed for small robots; they are not intended to bear more than a few pounds. With more weight than this, friction between the ball and the housing will probably keep it from rolling well.