Hi Judith,
You will find rain ponchos for sale in the resorts and theme parks in abundance the second the first rain drop falls! Every store will stack them by the register, and they'll go like hotcakes.
However, I always recommend tucking your own into your purse or backpack before you arrive. Often, I carry the clear disposable ones that you can get at a discount or drug store for about a dollar. They don't offer significant protection in a torrential downpour, but can keep you dry in a lighter rain or long enough to dash from one building to another. Also, they are light and small enough to tuck into even a small cross-body bag.
If I'm at
Walt Disney World during the summer rainy season or when the forecast calls for substantial precipitation, I pack larger, sturdier ponchos in our backpack. Again, I like to get them in advance from a discount or drug store. Besides the convenience and cost savings, there's an advantage to having brightly colored rain gear from an outside source: You can easily spot your family in a sea of white Mickey ponchos.
Thanks for visiting the Disney Parks Moms Panel, Judith. We're always happy to help with tips to make your vacation as comfortable as possible. Enjoy!
Wilma