Only other option out there to mention is the bumper recycler places themselves. They basically get all the damaged bumpers from bodyshops, inventory & store what is worth repairing, & chop up the rest. When an order comes in, they pull the part & it goes through the repair process.
I know this from getting different unrepaired cores from a local place over the years & initially getting shown some of the procedures. Many have very minor damages & hot air plastic welding , which is the cheapest procedure material wise, is also the strongest. Personally, at our last local place, they didn't get into chopping 2 bumpers into 1 sort of thing, mostly rip& tab repair on decent cores. Was possible if asked to view part before repairs.
Of course these sort of shops are few & far between.
Other than that, check online for actual true Mercedes dealership parts stores & pay the price.
Just to mention, I dealt with someone directly off an e-bay ad (Todd?) that was a friend of the owner of a place called 5 star airbags.com. He was helping sell some of a bunch of odd shipments of parts they got in from places. In early 2016, I got 4 ea 2016 Camaro auxiliary led running lights, no issues I could find, that were matched up with led drivers with Mercedes part numbers. Said they had a crapload of Mercedes stuff. Hmmm?