As I mentioned earlier in this thread, the cost of buying the required materials can be prohibitive, especially if you're only making one knob. There's also the added complication of making a fixing for it.
It's therefore, often, easier and cheaper to have them made for you. I have absolutely no connection with Wim Jaegers, apart from a thoroughly satisfied customer. He makes really excellent reproduction knobs and he advertises on Malcolm Bennetts website here (about 3/4 way down the page)
http://www.valve.demon.co.uk/Wanted.htm
David