Rockauto lists 7 tensioner pulleys for the 2003 slk320 : $48.79 - $186.79
for the NA crossfire, 6 of them : $90.79 to 186.79

idler :
8 for the MB : $11.73 - 30.99
6 for Chrysler : $21.79 - $30.99

as you can see, it often pays to look for a part under MB rather than Chrysler.

( the MB slk320 and the NA Crossfire are mechanically the same )

the tensioner at $48.79 for the 2003 MB slk320 is this one :
More Information for ÜRO PARTS 1122000970

the idler for the MB at $11.73 : http://www.rockauto.com/catalog/more...311&cc=1417104