They are available on e-bay all of the time (including the chromed ones). I purchased a few sets as spares since the ML and the CLK are both outfitted with chrome rims and "star" center caps.

Here is a typical example:

http://cgi.ebay.com/ebaymotors/MERCEDES-BENZ-AMG-WHEEL-WHEELS-CENTER-CAP-CAPS-CHROME_W0QQcmdZViewItemQQcategoryZ33747QQitemZ8037333296QQrdZ1

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car