I snipped part of a post regarding dealer only parts and a good source for them. Unfortunately I forgot the poster's name but I think it was member 'Michael'. Here's what he had to say:
Best deal I've seen is thru my local (here in CT) dealer - 20% off list (and there are several "list prices out there; these guys seem to use the most reasonable) for most stuff, and 25% off on filters. And free shipping. Dealer is Carriage House @ 800.437.3361. Ask for wholesale parts when you call.
I've never used them personally, but if someone has a good experience to share, please do so as I am always looking for better "dealer only" parts prices.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi