5how you get it
The bags Hardmode bosses drop are the only source, the roll is rare, and Queen Slime does not carry it. Classic worlds are out entirely, since those bags only appear in Expert and Master. Pick whichever Hardmode boss you can kill fastest and take the extra runs. The same roll gives you Yoraiz0r's Spell, a working accessory that arrives alongside the costume.
the pieces in play
Yoraiz0r's Scowl is the part people actually wear, since it lays a band of shadow across the eyes and reads clearly at any zoom. Its texture is solid black, yet the game still tints it by your skin tone underneath, which you can only see by pulling the texture out with a resource pack. The Scowl and the Spell also ignore gravity once dropped, so they hang in midair where you left them.
sprite work and leftover names
The goggles are what the name says, a recolor of Steampunk Goggles with light reshaping on the worn sprite. Every piece also carried a placeholder internal name in the code until 1.3.5, and they were all jokes: the goggles were filed as Testokun, and the skirt carried a line about it being developer armor whether you liked it or not.