Don't know the diameter, but there are several aftermarket producers. In metal, wood, etc. Though install was pretty easy, I asked a local BMW repair specialist to do it - took about 30 min, tops.
http://www.customwoodshiftknobs.com/Knobs2.html
http://www.britishautowood.com/cars.html
Craven
http://www.customwoodshiftknobs.com/Knobs2.html
http://www.britishautowood.com/cars.html
Craven