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