If you don't mind spending the money all at once I would recommend Hotronicsski boot warmers, they are expensive but sports authority had them on salefor $100 this year, I have bought way over that in disposable ones.I talked a friend into buying them this year and he swears he can wear less clothing now. The packs are rechargable and hook right to the top of your boots I even found a similar item that runs on a 9 volt and am installing it on my riser handle of my bow for a heated grip.

edit- ebay has the next model up for $119

This is what Im ordering to use on my bowhandle Toe-sters