I would use Car-part.com but I would actually think that getting a new pair of headlights is the best thing to do, nothing like a bright light going down the road.
Campingworld will have these or will be able to direct you to where to get them. This is not a sprinter part but a RV part.
If you are near Elkhart, there are some used RV parts places that may have some used parts but again like the headlights, I would not buy used but only new.