Hi, Chas,
I might have an answer for you. I certainly have several positive suggestions.
A. Try:
http://www.daviesmolding.com/standard-products/plastic-knobs/pointer-control/default.htmlThey have something that looks about right (albeit in black) near the bottom of the page and they do a custom "molding" service.
B: See:
http://www.bobsbits.tv/ComponentCat.aspx?catid=22They definitely have something that might work, circa 80% of the way down the page.
C: See:
http://www.rocknrollvintage.com/prodimages/1966-Fender-Electric-XII-Dakota-Red-Refin.jpgThe Fender Electric XII has a pickup (rotary) selector knob that looks very similar to the item on the Antoria (albeit only in black).
If you wanted to, you could get some made to that pattern in whatever colour you want (I have an Electric XII to work from).
http://www.maplin.co.uk/Module.aspx?ModuleNo=1809&C=Froogle&U=1809&T=Module.
We would, naturally, have to very careful with the Fender knob...
D. You could just wait until Barry G brings out his version of the guitar and see whether he will be able to sell spare parts for it.
Hope some of this helps.
JN