Are you driving or flying/do you have use of a car?
What and how much are you looking to buy?
Time = money, especially at Disney. Most everything you could want is available at the food court. If you have the time and want to take a bus over to DTD and walk across the street to the Hess station to stock up on sodas, water, and snacks. But remember you'll have to carry them. Or there are several places just outside the DTD area if you have a car. Goodings is a high priced grocery store - but convenient. There is also a CVS or Walgreens and probably more than a few Publix to choose from. All the info on locations can be found using a store locater function on their websites.