I lost a couple and have no idea what to search for online.
I just came across them at https://www.kineticbikebearings.com/kbb9038-slotted-cable-guide-pack-qty-20.html, called Slotted Cable Guides or slotted (wedge type) cable guide.
The term slotted might help disambiguate from other search results.
They are possibly called "cable clamps". For most frames that I've seen, they looked like this:
When lost, they are usually replaced by zip-ties, which are a better solution in the first place, from my perspective.
Your frame, however, looks to use a different type of clamps which act like inserts. From the picture it is hard to tell whether the regular U-shaped clamps will work for you. For them to work, you need matching holes in the cable stop/guide, e.g.:
A zip-tie around the tube will definitely work in your case, but it won't be equally aesthetically pleasing to look at, to be honest.