Steampunk Hat

All three pieces come from the Steampunker, so the outfit is gated behind Hardmode and behind whatever it takes to get her to move in. She does not carry them permanently either. The hat, shirt and pants only sit in her stock while the moon is full or waning, which leaves you about half of any lunar cycle to catch them.

That is a real constraint if you play in short sessions, since a run of new and waxing nights can hide the set for most of a week of in-game time. Check the moon before you make the trip rather than after.

These go in the social slots and do nothing for your character. Dyed silver, the shirt and pants land very close to the outfit Alex DeLarge wears in A Clockwork Orange, which is worth knowing before you spend a rare dye on them.