First thing to do is ask the folks at the CMP shop if they can help. Sometimes they still have the parts, sometimes they don't, but that would be my first stop.

Ball bearings of every possible size are readily available for purchase at machine supply houses such as McMaster-Carr.  Over night delivery (<24 hours) is typical if you live near a major city.

Bicycle shops may have a supply of small loose bearings, too.

The size you need can be determined by measuring the remaining ball bearing(s) that you have.  Also, ball detents don't need to be a perfect size (1/8" versus 3 mm), they just need to work.

A small sample of what's available - Link to Ball Bearings

Yes, you will end up with 100 bearings but the bag of balls will only cost $5.