If you follow a guide all the way through (I used lanoiregaming.com before I found the one on here.) you usually get it on the last mission just outside Dr. Fontaine's office where you inspect the morphine. Me and my brother both got it there, it might have been coincidence but I don't no. (:
"Because he's the hero that Gotham deserves, but not the one it needs right now... and so we'll hunt him... because he can take it... because he's not a hero... he's a silent guardian, a watchful protector... a Dark Knight..."