Start with Mercedes themselves - they can give you exact part numbers based on chassis #. If they don't have it, try online, or start phoning around scrap yards.
Any mercedes truck dealer will have access to their own parts.
They also have a section for their own after market parts.
It can be difficult to get genuine mercedes, unlike daf etc. Where you can get good genuine and aftermarket parts very easily.
Where abouts are you? The dealers will have access to the entire parts network for Mercedes so if they don't have one they can get it from another dealer or contact the huge elc at the head office in tongwell in Milton Keynes.