What are these heat pads that some talk about for using as a hot wrap? I would like to get one but I don’t know what type of shop sells them. We don’t have those all in one stores like Kmart or Walmart in England. Can someone tell me what these pads are usually used for, so that I can go into the right type of shop to find one? Are they things you put in the microwave that retain their heat for a longer time than rice in a sock from the microwave, or do you plug them into the mains to get it heated up?