Hey Wax,
Firstly contact the official Virus dealers (retailers) for your country, and they will be able to put one on back order for you.
Failing that, contact these guys:
http://www.synthesizerservice.de/index.php?lang=en
They are the hardware branch for Access Music, and are based in Germany. I managed to obtain some spare potentiometers for my Virus from them a few years ago, so hopefully they may have some spare knobs that you can purchase too.