If you don't mind the hassle of microwaving it every day, you could get the Snuggle Safe. It's a disc that you put in the microwave and heat for about 5 minutes, it keeps heat for 10 hours.
http://www.snugglesafe.co.uk/heatpad.htm
I bought one at PetSmart but I haven't tried it yet. I'm thinking of sticking it in MY bed to warm my freezing toes on these cold nights!