I haven't found a Mercedes center cap or any other one that I liked that is a direct fit for the NA wheels. I'm currently using Lexus caps from the previous generation ES. Those are not a direct replacement, I had to grind them down to fit. IMO the time spend looking and fitting them was well worth the effort but I understand some may disagree and not like the look.....
If you want a direct fit, you will need to find something in the 57-58 mm range. All the Mercedes ones were 60 or 62 mm.
Let me know if you find a nice direct fit