I would call them cable ferrules, but that same name also applies to the round cylindrical brake and gear cables like
Your best bet might be to troll through ebay, or to search out classic bike parts resellers or breakers.
If you don't care about period look, then a set of more modern brifters could work for the brakes alone, and keep using your existing separate shifters.
These ones are part of the brake lever, and are generally hard to find separate. Plus there's a lot of variation in diameters across brands.
These are intended to take the strain off the very end of the cable housing, and reduce the likelihood for the cable outer to bent sharply as it exits the top of the lever. So they don't sit quite flat in the top of the brake housing.
Your last resort might be to mill something up from a block of aluminium. I've made a gear cable endstop and while it was many hours of fiddling, the result worked well for me. Assumes a level of mechanical competence and access to a lathe and drill press, and inordinate amounts of time and dedication :)