4an early prize that lasts
The bag it falls out of can belong to the very first hardmode boss you kill, and nothing you craft for a long stretch afterward beats it. That makes it one of the bigger luck swings in an expert run. Get it early and yoyos stay your main weapon well past the mechanical bosses. Miss it and you are farming bags for a chance at it anyway.
how to chase it
It comes out of the same developer vanity pool as the rest of Lazure's kit, from any hardmode boss bag other than Queen Slime, so the fastest route is whichever boss you can resummon cheapest and kill fastest. If you already own Red's Throw there is no mechanical reason to keep farming, since the two behave identically and the difference is the sprite.