If you're looking for the ignition lock housing, this eBay listing Crossfire Ignition Switch Cylinder Lock Housing 04 08 New Original | eBay shows a manufacturer part number of 68001655AA. This looks identical to the one I installed on our '04 Coupe. The Mercedes part number W0133-1607413 for the housing/switch assembly gets you much better prices; the salvage one I installed came on an '02 SLK steering column for $80, including shipping.
If you're looking for the lock cylinder (has the tumblers, key inserts here), this one comes close Ignition Lock Cylinder Febi Fits 1997 2004 Mercedes Benz SLK230 SLK320 SLK32 AMG | eBay and lists part number W0133-1619200.

Hope this helps.