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.

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.

I have seen NOW brand egg whites in New Seasons, and Whole Foods. (In the Portland, Oregon area)

Where do you live? It would be easier for someone to suggest a local store.

I have found powdered egg whites (Deb El brand) in the baking aisle at Harris Teeter. Their only ingredient is pasteurized dried egg whites.

I am in the San Francisco area, and I have purchased Deb El brand from Whole Foods.