Amazon has a nice discount on the single serving packs of Horizon Organic Milk. Right now you can pick up an 18-pack of 8-oz cartons or Horizon Organic Reduced Fat Chocolate Milk or Vanilla Milk for $14.02 with free shipping when you buy it with Subscribe and Save. That makes each carton about $.78.
I love that this milk is shelf-stable, meaning that it doesn't require any refrigeration until after it's opened! I'm thinking about picking up a pack or two (with my Amazon gift cards from Swagbucks) so I can always have milk with us in my diaper bag. $.78 for a serving of milk is much less than I'll pay if we're out of the house for the day and I want my kids to have milk with a meal.
Need soy milk? 18-packs of Silk chocolate soy milk cartons are $17!