those chemical handwarmer things? You can get them in camping supply stores. The one I have (which I actually got at a sex toy party lol) has a little disc that you "snap" to activate. It forms crystals and heats up. When you're done with it you boil it and let it cool, and then it's ready to use again. Something like that would be portable- just keep one in your bag, and you could use it in the car, at work, or where ever.