Theratherm Moist Heating Pad. It's expensive, but it's WONDERFUL! It has a timed shut off so you don't get burned when/if you fall asleep. Moist heat works much better for me.
Electric heating pads will last longer, and as zaylil said, walmart should have one, and if not, there is always E-bay.
Good luck
Walmart sells a pad that lasts for hours called a thermapad or something like that. It's in the pharmacy section that sounds like something you need. Good luck also check online at like ebay and stuff and see what you can find but that would be my first suggestion.