A quick google found these, designed for animals, am sure there are plenty more out there, noticed these have chew proof cables though, maybe more suitable than a domestic electric blanket?
TOMO, do the neighbours know you've tapped into their electricity?
Here's another type that you microwave, looks like you get a free cat for the dog to play with too ........
http://www.bitsforpets.com/snugglesafe-heat-pad-with-free-cosy-cover-p-1556.html