The Pololu Half Inch Plastic Ball Caster kit includes a black ABS housing, a 1/2" diameter plastic ball, two spacers (1/16" and 1/8" thick), and two #2 screw sets. The total height of the ball caster can be increased from 0.53" to about 0.7" using the included spacers. The two included #2 screws can be used to mount the ball caster to your robot chassis (the distance between the screw holes is 0.58").
Note that this ball caster is designed for small robots and is not intended to bear more than a few pounds. Otherwise the friction between the ball and the housing will probably keep it from rolling well.