Ohh ... coincidence my wife made a silicon mold these week-end to reproduce these knobs as i needed some.
But we need to make a new batch of epoxy resin to try it again as the epoxy ended being not sturdy enough (Critical fail of the mix).
If you can't find a source, once we've confirmed the new batch is good i can ask my wife if she can make some for you. But knowing her she'll ask some small $$$ for this