Royal Tiara

The Princess is the only source, and she is the last resident to arrive. Every other town NPC has to be alive and housed at the same moment before she moves in, which in practice puts her well past Plantera. Lose a townsfolk to a Blood Moon and she can leave until you replace them.

Once she is settled her stock never rotates, so the tiara, blouse and dress sit there for whenever the gold is spare. There is no event or sky condition attached.

The outfit is close enough to what the Princess herself wears that a player in the full set reads as a matching pair beside her. It is a social slot costume with no effect on how you play, so treat it as a build piece for a throne room or a roleplay town rather than something to chase early.