Pololu Ball Caster with 3/8" Metal Ball
This ball caster kit includes a black ABS housing, a 3/8" diameter metal ball, two spacers (1/16" and 1/8" thick), and two #2 screw sets. The total height of the ball caster, 0.4", can be increased to about 0.6" 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 0.53").
Note that our 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.