http://www.petnap.co.uk/acatalog/pet_heat_pads.html#a30Scroll down to the flexiguard. The safest I've ever found, not chewable like the plastic ones. And I've had mine for years. You can get a fleece cover, but I don't sew, so bought them. I'm sure that most domesticated people would make their own.
Would highly recommend, and I've tried just about every type going. :-)