Everywhere contracts with either Coke or Pepsi. Thank goodness for me, WDW is Coke, 'cause it's the real thing. I guess they may have Mr. Pibb (my kids like Pibb or Pepper) but I don't remember, maybe not even that.
If you are driving I would go by the grocery, and if you are flying I would call GardenGrocer to deliver some. In the parks, well, if there were no Coke, I would drink water, but I usually do anyway.