Reza,
I'm not sure if you can find one that is pure wood/wood covered without going custom or to a tuner. That being the case either Brabus or Renntech should be able to hook you up with some truly beautiful pieces. You should be able to find a MOMO wheel with a smaller diameter but you will lose the airbag and perhaps horn. (and it will not look very classy)

One other possibility would be raiding the corporate MB parts bin. Though I don't know offhand whether it would readily swap a Sportline or other model steering wheel that has a smaller diameter than your stock one might be something to consider. Submit a parts inquiry and we can check it out for you.

Hope this helps...Lee