I have 2004 Sprinter, on mine there is a plastic part that slides in the track and is held on with a bushing. If it's that piece you're talking about, you can't order it, you have to buy the whole assembly.
It seemed wasteful and expensive so I I recovered the broken plastic part from inside the door, modeled it and a coworker printed a couple of 3D parts for me. We drilled out the bushing and used a binding barrel post (purchased from McMaster Carr) with loctite to install the new plastic part.
I was so happy with the part I bought my own 3D printer. Now I can make my own parts. I've made parts for a friend and a few other parts for myself.