Around Easter time, local feed stores normally sell chicks of various breeds. The chicks are anywhere from a day old to a couple of weeks old. Pullets and straight run chicks can be chosen. Pullets are female chickens which grow up to lay eggs, while straight run chicks have not been sexed. Chicks from feed/farm stores usually have a minimum amount of chicks that you have to buy (normally around three or four chicks). This is necessary so the chicks can keep each other company and warm during the trip home. Chicks from local feed stores come from either a hatchery or a local breeder.
Chicks can also be purchased online. Shipping is usually expensive, and you have to buy more than one chick. Some chicks do not survive the trip because they are shipped shortly after they hatch. How do they eat during shipment? They don't. The yolk that they absorbed during incubation can sustain a chick for 24-48 hours after hatching, so that is why hatcheries and breeders prefer to ship chicks when they are newly hatched. Chicks are shipped in bulk so they can help keep each other warm.
Online hatcheries and farm stores sell chicks that are NPIP certified. That means the chicks are not carrying any diseases.
I have also bought chicks and adult chickens from local people, breeders, advertisements, and the flea market. Most are not NPIP certified, so I just keep those chicks/chickens separated from my NPIP "clean" chickens. I prefer to buy mine local because the chickens are normally not shipped but raised at home.
After buying chicks, they need to be taken directly to their brooder box and heating source. The longer they are away from heat, the more they are susceptible to getting chilled and dying.
If you are buying adult chickens or juveniles (chickens that no longer need heat), they need to be separated from your flock to make sure they are not sickly or carrying diseases that could spread to your flock. I normally quarantine new chickens for 2 to 4 weeks before introducing them to my flock.