If you can find them here the White Bunny Boots from Bata are the warmest boots I have ever worn.
They are a little heavy but I don't think you'll find a warmer boot.
These are the Arctic boots the US military use.
Keeps my tootsies toasty @ -30 while sitting in my hut ice fishing.
I got mine for about $50 US while I was in the states.
Lots to be found on Amazon.com and ebay though.