There is a product in the pet stores here that's called a heat reflector -- althought I've noticed it I'm not sure quite how it works, or how much it costs ... It looks a little like those metallic looking blankets that they call space blankets? (I feel like I'm babbling -- has anyone else seen these things?)

Another idea.
My cats enjoy my 'corn bag'. It's like a little pillow filled with SEED corn (NOT popcorn) that I heat in the microwave for 5 minutes prior to taking it to bed as a foot warmer. It's rather like a hot water bottle -- stays warm for 6 - 8 hours. They're sold many by charities here at craft fairs, but I imagine you could find them on the internet???