Julie,
Each of the theme parks has a
Baby Care Center. These centers offer quiet, air-conditioned settings where you can change a baby, watch some Disney Junior shows in a little play area where they can also play with some age-appropriate toys, or you can even nurse in one of the private nursing rooms. This is available at no charge, by the way.
They also have small stores where they sell the baby basics, like diapers, wipes, over-the-counter medicines, and also baby foods and formula. I would not call their prices bargain-basement, but they are not too outrageous. The selection is limited, as they don't sell those here as a revenue center so much as a service and convenience for parents, but you should find enough to keep the little one's tummy full.
You will also find some selections in the food court/quick service restaurants of the resort hotels, including ones that are just a walk, boat, or monorail ride away from three of the four parks.
Write me back if you have any follow-up questions.
Have a magical vacation!