5how you find it
Kazzymodus' clothes ride along inside hardmode boss treasure bags, which locks the whole outfit behind Expert or Master difficulty. Every hardmode boss can drop it apart from Queen Slime, and the chance is small, so expect to open a great many bags before the hood, chest and legs have all turned up. There is no recipe and no shop selling any of it.
what it is for
The three clothing pieces are cosmetic, meant for the social slots where they cover your real gear. The set also brings Kazzymodus' Wings, which are a genuine accessory and should not get filed away with the costume by accident. The outfit belongs to the person running quality assurance at Re-Logic, and it is one of the newer developer kits, added well after the original batch of them.