http://www.baffin.com/
These are designed for polar explorers. They wick very well and I find them toasty and cozy as slippers. I use the Vermonts for general freezing temps and have a -40 rated boot for the extremes. The neg 40s are bulkier. I haven't had to try them out yet. After the first ride with the VTs down to about the teens I never again gave a thought to my feet other than... oh! That's just delicious!
winterfootwear.com and something called 6pm carry good sales.