I'll confirm that these guys have access to MB's inventory. I ordered a car cover over the phone from Rusty Cullens (***************) and it was drop-shipped from a Mercedes warehouse.
There's a certain large distributor that sells the web parts program used by a lot of the independent sites. Orders through the website have to be fulfilled by that distributor. If you can't find the part you're looking for, call them (FastLane, BuyMBParts, etc). The guys that specialize in Mercedes have other suppliers, including Mercedes-Benz.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino