Organic Valley and Horizon both make self stable milk available in individual servings. I've used both when we were on a boat and they worked well. I've seen them at Costco, Sam's, and Walmart. Here is a link to what I am talking about.
http://www.organicvalley.coop/products/milk/shelf-stable-milks/You could also try an alternative, like almond or soy milk. Many of them come in cartons that are self stable.
Good luck