You can buy baby food in the parks, and maybe even at the hotels, but it will be more expensive than it would be at the grocery store. Instead of doing that, I would suggest one of these 2 alternatives:
1) You could order it ahead of time from an online grocery store and have it shipped to your resort. It will be waiting for you when you get there.
2) You could get it from a local grocery store when you arrive. If you're renting a car, it won't be hard at all. If you're using a shuttle service to get from the airport to the hotel, it will take a little more planning. Tiffany allows for a grocery stop, so you could do it that way. Another alternative would be to call a cab once you arrive at your hotel. The closest grocery store is Goodings. It's on Disney property, so it's still more expensive than a normal grocery store. Publix or Wal-Mart would be the best choice cost-wise.