I could be wrong, but I get the impression that the OP is referring to what is commonly known as an Italian soda, not an ice cream float or ice cream soda. Like I said, though, I'm not entirely sure.
In any case, if anywhere on property does indeed make a chocolate beverage using carbonated water, be it with or without ice cream, the Fountain, Beaches and Cream and Ghirardelli are probably the best bet to find them.