I'd suggest that you forget about sourcing keytops, and just find/buy a complete typewriter. Without being able to perform an inspection of the typewriter yourself, do you know that it's only the two keytops that are missing and nothing else? For example, there's someone in my area who is selling a Rheinmetall that is not only missing a keytop, but also the entire typebar - and is asking an obscene amount of money for what is essentially an unusable typewriter. I would fabricate a keytop, but a typebar with its specific metal characteristics is a far more difficult project.