You should consult with your child's doctor about formula. (you probably will of course) You don't want an unhappy tummy if the formula isn't right. If you have an older child who has used formula successfully, they may suggest that brand. Some children cannot use certain brands.
You might consider having a few weeks of formula under her belt before venturing out.
If you are not having problems with breastmilk, consider waiting until you return home. WDW is very baby friendly in all ways: formula and feeding stations very nice. It is acceptable to feed baby anywhere of course but the quiet and relaxing baby areas help baby and parent better.
Hope all goes well for you! A baby is wonderful to take on holiday!