That part was never available for separate sale. it is trapped between the metal side plates of the motor truck. As far as I know, to replace the part, you would need to un-stake the metal parts, and re-stake them, keeping every thing square.
That part is common to several motor trucks. Which one do you need to repair?
New, old stock ALCO trucks are readily available and run $10-$20 (less armature and brush plate).
You could always buy one of those, and break it down to get the part you seek.
