We've always gotten it from Garden Grocer. I've never seen it in the drink areas at any of the resorts. I did read in a thread that some of the sit-down restaurants might be able to provide it if it is mentioned at the time of booking an ADR. We've never tried that so I cannot tell you for sure.
As the mom of a lactose-intollerant kiddo, it sure would be nice to find a carton of it somewhere on-property!