I use plastic 2ltr coke bottles filled with warm water (92 deg on my infrared) to start with, the need to be changed out every couple hours. After the hot water heater runs out of warm water I switch to hand warmer type heaters.
But because of expense of the warmers and the fact that the power isn't normally off for more then a day this works good for my collection.

http://www.bigappleherp.com/Heat-Packs