Recently, The Church of Jesus Christ of Latter-Day Saints changed their powdered milk shelf-life estimates. Recent scientific studies showed that powdered milk can be stored for up to 20 years (depending upon storage and packaging conditions as well as the quality of milk). 20 years gives you a long time to rotate through your powdered milk.
There are a lot of different companies that make powdered milk. Most stores even offer in-house brands, though these are usually sold in boxes. I would strongly suggest that you buy powdered milk in #10 cans rather than in cardboard boxes, which are poor containers for storage. If you like a brand that comes in a box, make arrangements to use a canner and transfer the milk into #10 cans.
Some common brand names of powdered milk are Provident Pantry, LDS Cannery (not really a brand name), Morning Moo, Carnation, Country Cream, and Country Milk. The prices vary widely from around $8 to $18 per #10 can. It's important to taste the milk you're storing. Brand preference seems to vary widely. Which milk you prefer will likely depend a lot on the kind of milk you normally drink. To save yourself from buying a lot of milk, only to discover that you hate it, share samples with your neighbors or start by buying only one can (small if possible) of any brand of milk.
I've tasted most of these brands and personally found Country Cream to be most similar to the 1% milk that we drink. Country Cream is more expensive than most powdered milks. I buy it, though, because I know that my kids will drink it. I don't like the cannery milk, but know some that really like it. You're lucky if you fall into that category, because cannery milk is also one of the cheapest powdered milks. If you find that you like Morning Moo, you need to be aware that it is a milk ALTERNATIVE. They've added sugar and partially-hydrogenated vegetable oil (trans-fats) to it as well. I like the taste of Morning Moo, but would hesitate to cook with it.
You can also store evaporated milk or shelf-stable milk to supply your milk. These are both more expensive alternatives.
If your family doesn't normally drink powdered milk, but you want to use it for your three-month supply, you can mix some and add it to your regular milk. You might start with proportions like 1/2 regular milk and 1/2 reconstituted powder milk. If you gradually increase the proportion of powdered milk over time, it may help your family to get used to a different milk taste. Powdered milk can also be added to basic baking recipes like bread, pancakes, oatmeal and sauces. Because of the longer shelf-life and by using powdered milk regularly, you should easily be able to rotate your powdered milk.