There's no Dr. Pepper on Disney property (they sell only Coke products). You can, however, get chocolate covered bananas in every park (they do include nuts)
If you're not a Dr. Pepper purist, they have Mr. Pibb (or Pibb Xtra) at several locations. Earl of Sandwich and Mara are two places I can get my Dr. Pepper fix, and I'm sure there are more places. Mr. Pibb is Coke's version of Dr. Pepper.