TSC usually has Leghorns. Those will be the white egg laying champs. Brown egg laying champs will be a Red Sex Link or ISA Brown, depending on what your local store gets in and where you live in the USA.
Good layers, with larger bodies will be Black Australorps, Barred Rocks, Black Sex Links, Rhode Island Reds, etc. Because these birds come to TSC from hatcheries, it will be unclear as to just which strain of these breeds you will get. It is not unlike buying a "pig in a poke".