As
@DisneyJoe said, the resort gift shops carry 1/2 gallon milk and liter sodas (etc) for sale. Just look in their fridge section.
If you tell me the resort, I may even be able to tell you where to look!
The only place I can't recall seeing it (though I'm sure it's there) is Pop...but I'm fairly sure they have it too.
If you don't want to pay their prices (which I don't recall them being vastly unreasonable) the 3 BP stations on site should carry them as well. There is one near the Car Care Center, one in the middle of the property (can't recall the name of the road right now) and one across from DTD within easy walking distance.
Also, there are food delivery services that will deliver groceries to your resort hotel if you'd rather do that.