I will swap my OEM leather 1990 300SL and airbag somewhen in the near future...It is the exact same steering wheel as was used in the W201 and early W124s. You can even get it with airbag...
Otherwise you might want to try
http://www.mbatc.de (mercedes original and own web site for outdated/new/used/over production parts).
greetingz,