I can't claim that I found these, as I read about them on Wombattle's web page documenting his Iron Butt Rides.
http://wombattler.com/I bought some "Hotteeze" for Feet and tried them out on the Border Run. They are a one-shot sticky pad that you stick onto the underside of your sock. When the pack is opened and exposed to air, the contents undergo an exothermic reaction and give off heat at around 37 degrees for quite a few hours. My feet stayed warm for the four hours it took me to ride from home to out near Iron Knob on Friday night., and were still cranking the heat out after six hours.
(there can't be much fresh air inside my boots) Generating a temperature of 37 degrees, they do not feel particularly hot, but very effectively stop your feet from getting cold. A pack containing five pairs of Hotteeze for Feet will cost you just $10 plus shipping. I bought two packs (10 pair) and Hotteeze tossed in another pair.
See
http://www.hotteeze.com.au/ for more information and ordering.