I can get powdered egg whites at my Safeway, in the baking aisle near the alternative flowers and the baking soda.

My King Sooper doesn't carry them. So, I'd just check a selection of your local supermarkets. A quick google shows that some people use them for protein shakes, so a local HFS or vitamin type store might have them also.

They do give the end product a certain texture, but the DH didn't notice at all, and it doesn't bother me a bit.