Milk provides calcium and protein, both of which are particularly important for growing children and nursing mothers. Milk also adds flavor to many recipes and sauces. Evaporated milk, sweetened condensed milk, shelf-stable milk, chocolate milk or cocoa mix (with milk included), and/or canned cheese, can also be stored to provide calcium needs. These other calcium products, however, have shorter shelf lives and need to be rotated regularly.
Researchers at BYU have found that powdered milk can be stored in #10 cans for up to 30 years in ideal conditions. Non-instant and instant powdered milk both store well long term. Powdered milk costs anywhere from $7 to more than $18 per #10 can. Milk can also be purchased in cardboard boxes or other containers. If you purchase it this way, you'll need to transfer the powdered milk into PETE containers and add an oxygen absorber for long term storage.
Make sure that you taste the brand of milk before you store large quantities of it. LDS home storage milk is the one of the cheapest brands ($7), but many people (including me) don’t like the taste. Don't know which milk you like the best? Have a powdered milk tasting party/enrichment. It's a fun and inexpensive way to sample the different milks without buying a large can of each.
I also recommend that you read the labels before buying large quantities of powdered milk. Some are actually milk alternatives and have added sugar and/or hydrogenated fats and less protein. Some kids prefer powdered milk made this way. However, these added/substituted ingredients can affect cooking and baking.
Here is an older post on powdered milk that talks more about specific brands and easy ways to rotate it from your longer-term supply into your three-month supply: More On Powdered Milk.