Frister & Rossman Cub 7 spool holder?

The spool which holds the cotton reel on my Frister & rossman Cub 7 is meant to be pushed down into the body of the machine when finished using it. The spool is held in place by a small cylinder of plastic with a spring clip underneath. The plastic bit has crumbled and so the spool does not stay in place but drops down into the top of the machine. Where can I get another plastic bit, and what is it called?