If it's cold and going to be raining for much of the day, I recommend buying the Mickey ones at WDW. But for warmer weather or where it's a quick afternoon storm and then it's over, I use the ones that are 2 for $1 at Dollar Tree. For children, we just tie a knot in the bottom corners to bring it up off the ground, and we're all set. We use them, and then dispose them (or pass them on at Kali River Rapids). In the warmer weather the Disney ponchos are so hot & heavy. But, the insulating factor of the Disney ponchos is great when it's cold!
Four of the ones from Dollar Tree fold up to a very small package and are extremely lightweight, so it's easy to carry them around just in case. I would not want to carry 4 of the Disney ones around because they take up much more room and are fairly heavy.
As I said, Disney ponchos are great when you're going to be wearing them all day, but not to carry around.
I just replenished my Dollar Tree supply a few days ago, and they had yellow, orange, & blue (each with a clear one to make a set of 2).