You CAN buy just whites in containers (should be by the cartons of eggs) but read the ingredients to make sure there is just egg whites in there. They are really good for protein at about 17 calories each and 3-4 grams of protein.
They are good scrambled or in an omelette with veggies, but if you mind the whiteish color, try adding one yolk in. Egg white and salsa omelettes are really low cal, high protein, and yummy!
You can also use them in baking in place of eggs- usually I think its 2 egg whites for 1 whole egg.