I read about some that have a center that you heat in the microwave, and place back in bag, it's said to keep the heat for 6 or so hours, you'd have to do a google. Here's a link to the tradional kind that is said to be safe for pets.
Heated Pet Bed Warmer - Three Sizes: CozyWinters
Oh here's a microwavable one, at the same place.
Microwavable Pet Bed Warmer: CozyWinters and it lasts for 12 hours.