17 Oct 2017 10:06
Guessing it's a sealed bearing? Otherwise just buy the same sized balls.
If it's sealed then pop it out and have a look at it, there should be a size on it.
You can service sealed bearings too if you can be bothered. Use a scalpel/craft knife to carefully lift off the seals, flush out the old grease and repack. Be very careful with the seals as if you bend then they'll likely never properly seal again. No good if they are rusted or otherwise damaged but if not it's a good way to save replacing them.
Vincenzo Nibali:
"I know how to ride a bike"
Reduce your carbon footprint, ride steel.