If no spares are available, could you make what you require?
Measure the diameter of the balls.
Possibly, once in place in the drawer runners, the plastic is only a spacer to, maintain separation of the balls, so maybe the balls to do not need to be completely captive. In which case, a suitably sized piece of plastic or metal with clearance holes for the balls could be used as a replacement? Fiddly to assemble, but better than nothing.
Howard