Where and how does Sunny get her juju knife, and what does it look like?

Anatov takes Sunny and the others to the Junk Man's shop. The knives are piled in a beat-up cardboard box. The Junk Man tells Sunny to close her eyes, reach in there, and pick up a knife. She does as she is instructed, and she is cut by one of the knives. The Junk Man reaches into the box and pulls out the knife that has Sunny's blood on it. The knife's handle is smooth silver, but the blade is paper-thin made of a clear green material, like glass. Sunny pays 13 coppers for her knife.