Once you get to -75F boots, you're looking at the heavy-duty Sorels (
http://www.sorel.com/ ) which the oil riggers use when they work in the winter up north ... or even around this area. You're talking one serious (and very expensive) boot.
http://www.sorel.com/Product.aspx?to...t=110&prod=127
http://www.sorel.com/Product.aspx?top=1&cat=110&prod=68
http://www.sorel.com/Product.aspx?top=1&cat=110&prod=51
I did a century a few years ago at about -25°F, and wore my Snowlions (similar to the Blizzards):
http://www.sorel.com/Product.aspx?top=1&cat=120&prod=58 which were fine for keeping my feet warm, but my feet tended to wander all over the pedals.
For temps that are not quite as cold as that, however, have you had a look at my "Cold Feet" page?
http://www.machka.net/whatworks/coldfeet.htm