Schaeffer products can be ordered direct, but requires a rather large ($300) order to get the best pricing.
I purchased a bunch of oil and Soyshield for Millie shortly after buying her. Included in the order were two cases of Soyshield at 4x1 gallon.
I experimented with Diesel Kleen and Soyshield in both the 300SD and my VW TDI and both vehicles responded better to the Soyshield, so that is what I have been using.
Now that I am actively trying to keep the miles off Millie, I should probably sell one case of the Soyshield or it will be in my garage for 10 years!
Here is a link to read up on Soyshield:
http://www.schaefferoil.com/soy_shield.html
IIRC I paid $88/4 gallon case in early 2007 (this is the discounted price when you order at least $300 worth of stuff). If anyone wants to try this stuff in the Houston area, I'd sell you a gallon for $20. I have 4 gallons I am willing to sell.
Tim