We got ours at the dollar store. I also saw some in the camping aisle in the Walmart sporting goods department for 99 cents. They are in a wallet size packet hanging on the accessories wall.
The more expensive ones at WDW used to be significantly better than the cheapie ones. Over the weekend, we bought both ponchos and an umbrella at WDW. The ponchos now are only marginally nicer than the 89 cent Walmart versions.