The best place to ask is the Westone Forum:
http://forum.westoneguitars.net/forum.htmIn fact if I remember rightly a member over there actually made, or had made some replacement brass knobs.
You may need to show a pic of your knob (sorry!
) - I had a Thunder 1 guitar and the knobs weren't tapered - just conventional parallel short cylinders, shaped pretty much like a speed knob. I needed one replacement and eventually got one through a different forum, which came from a Thunder 1-A bass - it was the same as mine. Yours may be from a later model.
Anyway there's a lot of spares trading at the Westy forum so somebody might have what you need.
Jon.