I've used Pelican Parts for years. Good service and competitive prices. However, being a Kalifornia resident I have to pay sales tax if I buy from them. Small items are not a problem but expensive items make me look outside of the State. MB of Atlanta is a good site to check on dealer prices. Sounds also like you need to sign up for EPC so you can find the EXACT part number for your car.
And one last word, don't buy cheap crap from eBay or other sources for critical items on your Mercedes. A sign in my garage says: "There is nothing more expensive than a cheap Mercedes"
Anziani
'97 Cl600 52K