Sunbeam electric blankets on sale at WalMart.com and BM. $25-50 for Full through King sizes. Read the reviews as Full and Queen will cover most of the top surface area of King mattress. Buying a smaller one will save on electricity used. Compare blanket dimensions to mattress dimensions here... http://en.wikipedia.org/wiki/Mattress#Mattress_dimensions
I searched FW and only found a post for smaller sizes @ $19.