Performance products sells it; I think the price is about $10/liter. I think George Murphy sells it - his company is Performance Analysis Co. I don't have the number, but you can find it in the Mercedes mailing list archives - http://www.mercedesmailinglist.com
There's a search function for the message archives, much like this site. Lastly, I bet Rusty carries it - 800-741-5252. I've been a dedicated Rusty client for several years now.