From what I know, there are three main suppliers of filters for MB. MANN, Hengst, and Knecht. You can find these filters in other places besides the Dealer...like here for instance. I used to put on about 1k miles a week on my 300D, so I was changing my oil every 3 weeks, in order to save a few bucks I bought my filters 10 at a time, and ended up paying almost the same had I gone with a Purolator or a Deutsch, except now I have peace of mind. The parts shop here has decent prices on filters, and they are the the ones that you would want to use.
-my two cents,
Larry