I lucked out--my neighbor's didn't take care of their practically new bubble pool cover, so it tore and they had to replace it. They gave it to me and I was able to cut the good parts into 9 spa blankets!
I have them rolled up and ready to use, one by one. I don't have my current piece cut to fit perfectly, nor is it long enough to overlap the tub when closed, but I am most pleased to have a bunch of free blankets to experiment with. (the next time I get out the scissors, I will have a better idea of how to cut it!)
I figure I'm set for a good 7-8 years! 8-)