Here's the 40 hr ones from Reptile Basics. I use these for shipping and keep a box of them in the house at all times in case we lose power during winter weather.

http://www.reptilebasics.com/40-Hr-D...k-p-16383.html

When this has happened in the past during winter storms I would just shake them up, wrap them in small towels/big facecloths and then tuck one into each enclosure right between the warm hide and the enclosure wall (2 if it was a really big enclosure). Worked very well to keep the snake's warm without letting them lay directly on the heat pack.