Try to purchase ponchos with some weight rather than the really light flimsy kind. The flimsy ones are easily blown about by the wind and tend to stick to you once they get wet. The ones they sell in the parks are a good weight, but I'm sure you can find that type in other places as well.
Walmart in the camping goods section, or any sporting goods store should have them. But you get what you pay for. Some places like the dollar store or Big Lots have some that might be cheaper, but they aren't much better than a garbage bag. Just feel and make sure the plastic has a bit of weight to it. You will be glad you did. In the children's clothing departmant you can usually find character rain coats with hoods that button or snap up the front and have a cinch capability for the hood and they work really well with little ones. Stores like JCPenney's and such would probably have rain coats for kids, too.
I got disposable adult and kid ponchos at Dick's for about $1.29 (a few years ago though). I also bought nicer (reusable) ones at Dick's for about $8-10. We take the nicer ones camping (Nike brand I think) and used the disposable for WDW.
Can't say for sure about right now, but I found nice Cars-themed ponchos in the boy's clothing section at Target last year. On an endcap with hats, wallets, belts, etc.
I actually JUST bought ponchos at Target. They were on the end of the racks with undies/t-shirts. The ones I got were Hello Kitty and Transformers so not actually Disney themed but they will serve the purpose!
I actually JUST bought ponchos at Target. They were on the end of the racks with undies/t-shirts. The ones I got were Hello Kitty and Transformers so not actually Disney themed but they will serve the purpose!