Check out this on Ebay:

OEM Mercedes W210 Cabin Air Filter CLK430 SLK230 CLK500 - eBay (item 280458280853 end time Feb-17-11 12:10:00 PST)

Shipping is a little high, but they took my lower offer which made up for some of it, plus I didn't have to pay tax. Got it in 2 days.