Milk is still sold by the pint, quart, 1/2 gallon and gallon in the US, so if you ask for a litre you're likely to confuse the clerk.
Most of the resort gift shops have quarts or half-gallons available; the DVC resort food shops have gallons. IME, the food courts normally don't sell bottles larger than a pint, and it is less expensive purchased from the gift shop.
Also, the Hess petrol stations have a large variety of milk products at good prices. There is one opposite Downtown Disney and another near the entrance to the Boardwalk. (Also one at the Car Care center, but if you don't have a car that's difficult to reach.)