I've had good luck with Warrantech: http://www.warrantech.com/c_auto.html
Give them a look first. Most of the Benz dealers sell Warrantech warranties but you may find an independent service shop that will sell you the exact same Warrantech warranty at a far lower price. As far as I know you cannot buy it directly from Warrantech but you may be able to find out from them who sells their warranty in your area.
Be SURE to buy it before you are within 1 month or 1,000 miles of MB warranty expiration.
I bought one for my wife's '95 E420 and will buy one eventually for my '98 E430. I think it's well worth the price.