You can score a Sunbeam king size heating pad at both Walmart and Amazon for around $11.00 (reg. $20). The pad is 12 inches by 24 inches and should cover most of your back.
Choose free in-store pickup at Walmart or spend at least $35 for free shipping.
NOTICE: Amazon prices are subject to change at any time. We cannot guarantee that this price will be the same price when you attempt to order this item. It is highly recommended that you confirm the price in your cart BEFORE you submit your order.