Aoibox 15.4 in. W x 15 in. D Electric Foot Warmer Heater Fast Heating Pad Cushion Mat in Gray, Machine Washable HDDB889 - The Home Depot

Made of ultra-soft microfiber, this heating pad is next-to-skin comfort and retains heat better than other fabrics. Wide opening allows feet to slide in and out easily. Just plug it in and it heats up in seconds to keep your feet warm.