Wal-Mart is most likely source on a regular basis. O'Reilly's when it's on sale. And you can get the 0W-40 European Formula at both locations also, which is the preferred oil per M-B. I'm referring to Mobil 1 products.

I've also found Castrol Syntec 5W-40 at Advance Auto on sale occasionally. This brand is recommended by VW for my 06 New Beetle and is specifically listed as a suitable oil for the 229.3 M-B spec.