Mercedes stocks relatively few parts for older cars. In fact, they sold off most of the inventory of SLR stuff to Vintage Parts several years ago.

When it comes to the classics, Mercedes does have Classic Centers in NJ and CA. Those shops are pretty small but they can either locate parts for vintage Mercs or arrange manufacture. In fact, if there's a vintage MB you're interested in buying they can either help locate or restore the car you want.

Make no mistake. You will pay dearly for those components.

Want a new white steering wheel for a 1963 230 SL it's not a problem.
If you're looking for something like a vacuum pump on a 1983 300SDL you're out of luck.