Anyone got an electric foot warmer.?

I'm talking about one of those big boot things, often in a furry or padded fabric, that you can put both feet into. They have electric elements built in to heat them up when connected to a mains outlet and some also have built in vibro mage. I'm thinking of getting my wife one for Xmas but don't know how good they are. Can anyone advise? And can any Brit tell me a good make and outlet to buy one? ( I know Americans often mean well but its no use telling me of a great store in NY or LA that sell them when I live in the UK)