I have noticed that randomly I will sometimes have a citizen move in that is wearing a costume. After looking at the costume list it appears to me that these are "buyable" costumes using tower bux. This made me wonder - is it possible to get all of these costumes by random move ins, or are some "buy only". I'd rather save my bux for "buy only".
From personal experience, even if a bitizen moves in with a costume you haven't bought yet, it won't be unlocked for other bitizens unless you buy it with tower bux.