I recently bought a pair of Muck Boots, following the advice of René Pleinair.
These boots should be able to keep your feet warm up to -20C. Not that I plan to go out painting in those temperatures but at least when it is -5 your feet will stay warm. And indeed they do work. They are not cheap but I guess you get what you pay for.
|"Walking the dog", oil on panel, 30x24cm, available|