Many will say to stick with the foreign made parts but I found that RockAuto carries the Moog rod ends which are as good as they get. Reasonably priced too.

http://www.rockauto.com/catalog/rafr...partnum=ES3317

Clicking on the "ES3317" verifies that this fits the SLK320 and SLK32 AMG; same as the Xfires.
Click on "info" to view part info.

Update: You'll notice that the Moogs are "beefier" than the common Meyle and Lemforder made replacement ends...

http://www.fcpeuro.com/Mercedes~Benz...004&m=20&e=630