The clips tend to break especially if the van is older. In order to seal things and prevent moisture from getting into the van interior, there is a small sealing ring on the exterior side. Like all things plastic, there are the Chinese knock offs and the real ones from MB which are guaranteed to fit. I seem to recall a thread from a British guy who sealed each and every clip with a sealing goop (Sika Flex?) rather than buy the replacements. You could scavenge from a wrecker or bite the bullet and buy the real deal from MB, painful though that might be.....
Hi - yes that was me but I did buy replacements as well as seal them. Someone else on the other thread suggested keeping the OEM ones in place and putting some caulk around the outside which is a great idea if you don't already have some rust.
I had some rust starting around the holes and a decade of dirt trapped with it
https://i.postimg.cc/nh0KV5sc/IMG-3123.jpg
So I took all the clips out, sanded any rust spots or anywhere where the paint didn't look 100% and then put some red oxide paint on them
https://i.postimg.cc/3wqgmvCs/IMG-5265.jpg
While I had the panels off I gave them a good wash with some sugar soap to get rid of any grease and hung them out to dry....
https://i.postimg.cc/fymjwRd6/IMG-0885.jpg
Before spray painting them with some special bumper paint
https://i.postimg.cc/pXBzk7JY/IMG-7878.jpg
https://i.postimg.cc/MH2VrJLt/IMG-0841.jpg
Here's a 'before and after' shot - I was really pleased with the result
https://i.postimg.cc/Jh0NsqJ3/IMG-0242.jpg
I had broken a few clips taking them out, and there were some missing to start with so I bought some new ones rather than try to rescue the old ones.
I put some Sika Flex on the inside of the disc on each one and then also sealed as many as I could from the inside of the van.
https://i.postimg.cc/1XG6kYhS/IMG-0551.jpg
Eventually it was all done
https://i.postimg.cc/yYCyL04B/IMG-9012.jpg
I did notice that the cheap eBay clips were much thinner plastic and much more brittle than the original Mercedes ones and I would suggest that if you're putting all this time and effort into it then the extra money for the OEM clips would be worthwhile.
That's not to say the cheap ones won't work, but if you decide to go that route then buy spares because it's likely that you'll break some when fitting.
It's not a difficult task, but it is a time consuming one which is best done in good weather with a big handful of patience
Good luck!
*edit* Just re-read your request for help sourcing them.
I don't know the Mercedes / VW part number (clips are the same on the VW Crafter) but here's a US eBay link. It's just something I searched for, so no affiliation with or recommendation for the seller - it's just to illustrate the clip you are looking for.
https://www.ebay.com/itm/20x-VW-Cra...=332027135886d4cc2b52ef7e485d82a5a8bd9b9f3a0f
*edit, again*
According to that eBay listing the OEM part number is 2E1867289C and when I use that as a search term I do get a lot of results.