Any BMW dealership will have access to them, the issue is that you will pay ~$180 for it. I scoured ebay last night and couldn't find anything you needed as well as the parts section on advrider.
If you have the pieces you can use a a plastic repair kit and get them together (only do one piece at a time to ensure good placement) and then run a layer or two of fiberglass or CF underneath or a mesh backing with resin to reinforce the piece. You will have to sand it and repaint but this will be the cheapest method if you cannot source a new/used piece. As this is not a structural piece a cosmetic fix like this will work just fine.