These air filters come two to a box. You need a set as the intake is a 'Y' into the throttle body.

Mann is OEM and preferred here. Check the filter inside your box. Often it is just a Mann filter in a re-branded box.

2003 Mercedes Benz SLK320 Base V6 3.2L Maintenance Engine Air Filter - W01331626643 - Air Filter Set - ES#1957821

http://www.ebay.com/itm/400829364468?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT (Crossfire specific, discontinued by Mann - use the MB C2861-2 as alternative)

http://www.ebay.com/itm/Mercedes-Benz-MANN-C2861-2-AIR-FILTER-2PCS-OEM-Quality-/321660126772?pt=Motors_Car_Truck_Parts_Accessories&hash=item4ae4700634&vxp=mtr