While there is nowhere on Disney property to get larger supplies of these items, every resort gift shop at WDW does stock some diapers and formula. You can also find these items for sale at the Baby Care Centers in the theme parks. These are a wonderful resource in an emergency, but I wouldn't rely on them for any length of time. The sizes and brands are limited and the cost will be more than buying them from another source.
I do have some other ideas for you:
- Mail yourself a package of diapers, formula, wipes, etc. to the hotel. This way you are less likely to run out during your trip. You can read more about how to do this
HERE.
- Order baby supplies from an online retailer and have them sent to your hotel. Read more about this
HERE.
- Have baby supplies, and groceries, delivered from an Orlando area retailer. You can read more about this
HERE.
- You could also take a taxi to a local grocery or drug store, but the cost of this would likely make this option more expensive than buying at the resort gift shop.
Good luck and have a great trip to WDW. The Yacht Club is FABULOUS!