Enter by J-11 in Yhoator Jungle.
You will be at 1 on this map:
Exit at 4, you will be back outside in the Jungle (at 4, just take a turn right then right again to zone into 5.
All the mobs there drop the key. Tonberry Pursuer drop the Flickering Lantern for thief's knife NM. You trade it to the ??? in the small room at G-8 to pop it!