You can try snugglesafe. I've read a lot of success with it. Here's a website:
SnuggleSafe microwave heating pads for people and pets - nothing to fill with water, nothing to plug in
Also you dont need to plug it in. I'm debating if I should buy one too for the winter. The cheapest I found for it was on this site:
Snuggle Safe Microwave Heat Pad - Dog.com