Guggs, you will hardly find a store that carries Rickenbacker spare parts in Europe (except for the UK maybe). But US dealers are allowed to send Rickenbacker spare parts and strings to international customers, now. So I recommend to send an e-mail to a few American dealers and ask, whether they can ship these things to you. You find the US dealers on this website.
If you e-mail me at
markushorn@doctorno.de I might be able to give you some further advice where to get the oven-knobs, a 620 a.s.o.
Viele Grüße in die Schweiz!