We love DL in the rain - less people in the park!
If I remember correctly, you can find the ponchos all over the place when it's raining, and they used to have kids and adult sizes (it's been awhile since we bought ours). You can buy cheaper, generic ponchos in the camping goods section of places like
Walmart or Target - but I think if you have to wear one of those things around all day, it's much more fun to have Mickey on it!
Have a great time!