Hello
Try those little heat packets that are air activated. They come in two styles, one has adhesive that sticks to your toes and the other you need to buy a insole pad for your boot (Probably $7 at Wal Mart), which the little packet then slides into the toe portion.
I like the one with the insole pad. Those that stick to the underside of your sock seem to move around when you walk and then end up in a position thats uncomfortable. The one that slides in the pad will always stay in place and they stay warm for several hours at least.
Make sure you buy the packets that are for feet and not hands since theres a difference with the amount of air required.

Happy Hunting
Dan