Are egg whites generally available at the store?
Many stores sell egg whites.  Organic Valley Liquid Egg Whites as an example.  

They can be both pasteurized (useful if you want to use the whites in a non-cooked application) and non-pasteurized.  

In my US grocery stores, I find egg whites in the refrigerated section, near the eggs in cardboard cartons, similar to those that cream comes in.  

Depending on your application, beware of egg substitutes; these aren't quite the same thing.