If you get hold of this month issue of Mercedes Enthusiast, there is a company advertisment that they carry zebrano wooden gear knobs for 34 British pounds (approx $55 US). The web site to this company is
http://www.eurocarparts.com. I have never visit their site so I cannot tell you how good they are. To top it off, they are located in the UK so I am not sure what they will charge for shipping.