Do you have an O'really (O'Reilly) near you? They usually carry the WIX filters I need.
I have not had much success at Autozoo (Autozone) or Pep Girls (Pep Boys) either.
But best of all, if you can find a small independent import parts store and get to know them. They will usually go the extra distance to keep these basic parts on hand. Especially if they are near several indy shops that maintain MB's.
If you want real service stick with independent business owners.
Chains have forgotten this aspect of business it seems.
They rely strictly on volume.
(Hint: If an independent goes this extra distance, don't cut his throat over the cost of a cup of coffee!) I'm not saying you would, just that I see it too often. By the way Fastlane is one of these independents.