Where does Heck Tate get the switchblade?

Nothing, but if anything it's the truth of the scene. The switchblade was Bob Ewell's knife. He took it from the crime scene, because that was the knife Bob tried to use to kill Jem and Scout. But Boo(Arthur) Radley heard and came at Bob, stabbing him with a kitchen knife. Jem was unconscious and Scout was pinned so it had to be Boo. Heck took the switchblade, so no one knew this. So Boo wasn't persecuted for saving their lives.