The Washer-spring,18mm is the lowest priced item on this Rear Hubs/brakes schematic.
Some items from this specific schematic: Bearing-ball,25bwd02c, Spring,brake, Drum,rear Brake, Spring,brake,rh.
The thirty-six Kaf620-p9f Mule4000 2009 Usa Rear Hubs/brakes component parts are all listed in this schematic.