Princess Hat

Goodie Bags are the only source, and they come off enemies while the Halloween event is up. Each bag rolls a single costume item, so the way through is to hoard bags during the event and open them in bulk once it ends. Two pieces makes this a shorter hunt than most, since there is no third roll left to wait on.

Both parts are cosmetic and belong in the social slots, so nothing about your defense changes when you put them on. There is no leg piece here either, which leaves that vanity slot open for whatever you want showing underneath.

The dress here shares its name with the one the Clothier stocks, so a search turns up two entries that are not the same item at all. Terraria does this in a couple of other spots, including the seaweed that summons a pet and the seaweed you fish up as junk.