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. 

On several machines where 1 or 2 key-tops were missing...I just went into my parts-stashes and found/used others from a different maker.

My blue Olympia SM7 recently came to my home missing its back-space key-top.

I grabbed a key-top from on Olivetti parts stash and I am happy with the results.  Figured I would not find a proper Olympia key-top (for cheap) in my life-time.
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