My 17 month old is lactose intolerant, and I usually get her milk from Giant Eagle..... I usually buy flavored (vanilla/chocolate), and she loves it....
I've heard Sam's Club sells Soy Milk in balk, which are the one's that can sit out until your actually ready to use them.... But, once you open them they have to be used usually 5-10 days (depends on brand)... Giant Eagle also sells these types, but not in bulk...
If you want to buy plain (which is usually the same price) just add flavoring)....
Also, beware of foods that you feed your child... Read all ingredients, I learned the hard way... Spaghetti Sauces, Raviolis (canned), Hot dogs, Chicken Nuggets, everything... If it has any type of milk product (cereals too), they've gotten better at labeling but not all companies have!!!!
Some do outgrow it while others do not... My Daughter has not, but My Niece (whom is now 27 mos.) has completely....
Hope I've been some help... If so, and you need to know anything else feel free to ask....