I use Whitewoods cross country ski boots. They're well insulated, windproof and waterproof, and have kept my feet warm on winter rides down to single digits below zero Farenheit for over a decade. They're affordable, and if you don't mind the little duck bill thing sticking off the toes, they work perfectly for winter cycling. The tread is even a soft winter compound that grips pedals well in very cold and slick conditions.
https://www.amazon.com/Erik-Sports-W...a-888159881915