I have bought 2 spa blankets - one from Doc and one from one of his on-line competitors - Doc's wins hands down!
But I stopped using the blankets because I couldn't see spending $70 for a blanket to last 2 to 3 years so that I can get another 3 or 4 years from a cover. The cost for the blankets were slightly less than a new cover. I do plan on buying a new cover from Doc when the time comes and he does double wrap his covers so I see it as a win/win...
But if you need a blanket, here you go:
http://www.rhtubs.com/store/float.htm