I have been struggling with my local Body shop to complete the repair on my 300CD. They have struggled with getting the rear Quarter Window Regulator working properly, to no effect. My problem is related to this - they have damaged this interior panel to the extent that I will need to replace it. Where can I get one of these, new, if necessary?

That is not a part that we stock. We can special order it from Mercedes for you but, it will cost you the same as Mercedes list price.
For new, the dealer is about your only choice.
You may find one used but, the chances are slim.