They could be Phillips dual color mini LED lights. Target currently has a few versions of the strings and the specs list 3mm LED. The exact names on target are “Philips 210ct LED App Controlled Christmas String Lights Dual Color” or “Philips 200ct LED App-Controlled Color Changing Create Motion Dome String Lights” You could probably use the bulbs in your current lights, but the strings are kind of pricey.
There is another maybe from Sylvania that doesn’t list the bulb size. Amazon has “Sylvania Stay-Lit Platinum 100-Count Color Changing LED Mini Dual Lights”. Apparently Costco also used to sell these, but doesn’t have them right now .
It might not be possible to find just a few replacement bulbs, but I wish you luck.
Wow! What an interesting read about someone I’ve never heard of, but whose work has impacted daily life in so many ways. Its amazing how many systems rely on accurately telling time and the intricate solution that NTP is.