Powdered milk is made by spraying it in a fine mist in a tall tower, and it dries on the way down. I can't think of a good way, or any way, to make it at home. The shelf life is not that great, usually just a couple of years.
We have gone to a milk-substitute made by Honeyville Farms. http://www.honeyvillegrain.com We sure like the taste MUCH better than any other powdered milk product we have tried, and over the years I have tried quite a few. It is a whey product. It comes in large cans, six to the case, and it much cheaper to order from the source rather than a reseller. By about a third or more. Give it a try.
G