Does make one wonder whether peeps rip off buttons to resell. Let's hope not.
Extra button, and thread is supposed to come with the rtw pieces at time of purchase. However, that is not always the case. If you do a phone order and don't receive the plastic zip containing the extras, you should immediately call and request it. Ya never know when you might lose a button.
Back is the day and it may still be true that Chanel could locate a button--I'm guessing from the repair center, but there is a charge.