Craft stores in the baking aisles - like a Michael's or a Joann. They will also have meringue powder but that has extra ingredients. Try to find the straight egg white powder, they should have it, or specialty grocery stores, but those craft stores with the cake decorating stuff have surprising items sometimes.
you might try some of those end of the world survivalist sites, i know they sell foods that dont rot and keep unrefrigerated for years. i bet with a little research you could make your own.
I am in the San Francisco area, and I have purchased Deb El brand from Whole Foods.
I have found powdered egg whites (Deb El brand) in the baking aisle at Harris Teeter. Their only ingredient is pasteurized dried egg whites.