I was there on property for a few days last week. They didn't have the half gallons at any of resort stores and when I asked at PPH, the CM said they were out resort wide.
If you're driving in, there is a mini mart at the Chevron on the corner of Ball and Harbor that sells the half gallon milks for a reasonable $2.59... yes I trekked out there in between rain a week ago. The Shell kitty corner from Chevron only sells 16 oz bottles for 2.50.
At Craftsman Grill, they had 14 oz Fair Life for $5. A pint at Tangeroa Terrace is $2. Starbucks sells milk, venti or whatever is bigger than Venti for $4-ish