Sexing day old chicks is a bit sketchy, so if you buy again, just be aware. Even those that are "sexed" at hatch still have a 90% accuracy rate, thus, one in 10, on average will still be a cockerel. If you buy chicks from a "straight run" you'll get a yard full of cockerels.
The only way to be sure is to buy sex linked chicks. These are strains that intentionally made by crossing certain birds to produce the cockerels of one color and the pullets of another color at hatch.