The problem with them usually is that they don't heat too much and still consume the batteries very fast.
Try if you can find chemical heating pads somewhere. The are small fabric "pillows" filled with some kind of an iron compound. Once you unwrap them and they get in contact with air, they get surprisingly warm and stay that way for hours. There are at least two sizes, one to put in the boots and one for gloves. They have saved my day on more than one occasion