I don't think so--probably not. All of the WDW resorts do sell the larger bottles--I'm thinking 1 or 1.5 litre. Still fairly expensive, by my standards.
If you want to purchase less expensive beverages on WDW property, the convenience stores at the Hess stations are a possibility. I never remember to buy anything when I stop for gas, but I'm told the prices are much more reasonable than official WDW locations.