This is probably a question better asked of an employee of the grocery store(s) you frequent. They should be found in the refrigerated section of the store that contains whole eggs.
Eggology is a brand that can be found in the USA. I've used it before for making omelets and mixing into cocktails.
Besides convenience, an added benefit of buying egg whites is storage lifetime. Egg whites sold in bulk should be pasteurized which can extend their storage life up to four months after opening. Compared to the two to four days that raw egg whites can last, this is quite significant.
I do not use them for baking since I find that the aggressive pasteurization denatures the proteins some. This can subtly affect the rise and stability of some baked goods.