I would think most h/w stores don't have "ball bearing" by themselves and if they do not that small. Though, you should check it out anyways. If you have a loose bearing left, allow a machinist to check dimensions, etc. and see if a typical bearing can be broken to access the balls, What a good machine job or such offers if totally up in the air. You're going to have to scourg a bit before finding.
i used to be pretty good at refilling HP 45 black ink cartridges but i have run out of the ball bearings needed to seal the cartridges. seems silly to buy a whole new kit when i have all else here. can i buy the plugs or ball bearing plugs at a hardware store? any ideas? thanks ! - fj