I, too, buy mine from the Dollar Tree. I've used the same ones for two years, ballgames, theme parks, shopping. While at the parks, they're nice to have during the many rain showers that Florida has. I'm 5'8" and they come to my mid calf. As the other poster said, you get 2 for $1. I love them and always carry 2 in my glove box.
At night, I shake the excess water off outside then put them in the bathroom or over a chair, if they're almost dry, so that they are ready the next morning to be folded and put back in the backpack.