SparkFun ROB-08909 Ball Caster Metal - 3/8"
This ball caster kit from Pololu 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 this ball caster is 0.4" with no spacers. You can also adjust the height by adding the spacers.
The ball caster can be mounted to a chassis by the two screw sets included in the kit. The distance between the two screw holes is 0.53".