The best boots as for keeping feet warm were made for the military and are known as (Mickey Mouse) boots.
Probably originally a Korean war creation, made of rubber and well insulated.
They are not good for use by those doing much walking, but are the very best for those who sit mostly.
Still popular among many PA hunters who sit for long periods of time on cold days.
They come in two versions, one being black and the other white, the white ones are the best as for warmth.
They can be purchased both new or used in some Army/Navy stores such as Bobs Army/Navy
in Clearfield PA. 814 765 4652. Expect to spend about $100 for new white ones.