Skeeter is right. Boots must have enough room in them to allow proper circulation. If you have a long walk to your stand, bring extra liners / socks. When you get to your stand, remove your boots and socks, let your feet dry, put on a fresh pair of socks with a warmer in each boot.
With me, my feet are the first place to get cold. I have found that if I keep my trunk and head warm, my feet tend to stay warmer. If you will be on the stand for a long while, bring something to eat for energy. Keeping your feet warm is more about keeping them dry and keeping your whole body warm than just buying extreme weather boots.