http://www.cutlersupply.com/cart/index.php?main_page=product_info&products_id=939. That is a good price for 72 hours of warmth.
In a pinch, a hand warmer under the excelsior pad works ...8 hours at 135 degrees.
cool idea! We have some electrical hand warmers that double as usb plugs for your phone, etc... in a pinch. They'd do the same thing but I never put 2 and 2 together ;P