Hi mate,
Having had my Is200 since 2002 I went through about 4 OEM gearknobs, all of which worn down over a year or so and it used to drive me crazy.... until I found this on ebay: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=260755794883&ssPageName=STRK:MEWNX:IT
It is just like the OEM one only it's a SOLID polished stainless steel gear knob... It feels miles highter quality the the OEM one and costs about the same (I paid £39 inc. pnp). It's a direct replacement for the original and is easy to fit, just unscrew old one and screw on the new one.
The biggest bonus is that id does not wear as it is made from solid steel rather than just plastic coated in a thin layer of silver like the OEM one.
They come on ebay from time to time but the guy who sells them is on here (not sure who) I just messaged him through ebay.
Scott
Ps. Fantastic 'custom' version you got there Dave, whats that made using??