Hi--
In addition to above suggested vendors, I always check OEM Autohaus in Arizona, Rockauto (if they have the part, they usually have the best price by far), US Auto Parts, and eBay ( you never know).
Also I check the items for sale through our members and the for sale items in the US Mercedes Club.
My experience has been that shopping around is quite useful both for knowledge and for pricing. It is not uncommon to find a price range of 2 or 3 to 1 and up to as high as 6 to 1(!!) for the identical part from different vendors.
I'm a bit anal, and I make up a spread sheet listing vendor, part and price. Usually, I get some very interesting results.
Joe