No one will sell one chicken or chick on its own because it's a bad idea. A chick raised on its on will not only be constantly stressed and very loud during the process of brooding and raising, it is likely to be severely picked on and outcasted when added to the existing flock. The number one rule of adding to a flock is never, ever, ever add just one hen (the exceptions being roosters, and 1 on 1 introductions). I mean, sure, it can be done - I've done it in the past when I was a kid and couldn't have known better - but such birds are always loners from the flock and never integrate into the existing group. This also occurs when new groups of 2 or more chickens are added to the flock, but in that case it's OK because in each group the birds have at least one buddy.
