They come up now and again on eBay, around £100 per set. You may get more luck on eBay.de
They were standard fit on EU (AFAIK) C36's and some C43's, option code 776 "WHEEL COVERING FOR AMG 17-WHEELS (ONLY EXPORT)"
My C43 was supplied new without them in 98, Jon who now owns the ex-MB press C43; R733 JNH (had a couple of MBClub owners too I think) it was also supplied new without them.
For the 17's they don't do anything except hold dirt and moisture, but I understand why if you are running 18's they can protrude on the front.
Hi Notwen, just thought I would mention that I managed to pick up the set on UK Ebay this morning. £80.00 for the set of four in good condition and not far from where I live.