Welcome DWG, you will struggle to find them, this may help:
http://www.glcforums.com/forum/106-mercedes-benz-glc-general-discussion/4186-mudflaps-glc-43-a.html
http://www.glcforums.com/forum/106-mercedes-benz-glc-general-discussion/4186-mudflaps-glc-43-a.html