They are not available as a spare part and I doubt you'd be able to fit one from a used bearing as the plastic gets very dry and hard from the oil. They also shrink and I'd say that's how yours lead to being broken.
I would say the best way forward is to replace the bearing. You'll also need to change the outer race as the bearing is a matched assembly. Be sure to use a puller on the outer race and don't use heat.
