The Mk3 knobs are the same ones used on the Hapax, aren’t they?
Loads of Hapax users have been changing their knobs (there is a thread about it - Encoder switch issue - #38 by cowboy) so maybe you could ask people there if they’d be willing to send you their old knobs? Otherwise ask Squarp where they got them from with their contact form.
A lot of people don’t like the Hapax knobs and think they are cheap and plasticky, although I am not one of those people. The new ones are not rubberised, so not soft to the touch, and they are hollow inside, rather than solid, so I do get what people mean, but I am happy with them and how they look (although you need to wedge some paper or something in the end to stop them sliding down too far and preventing the push the click)