They drop from Insane Map treasure chest.
For treasure map system, they go like this roughly.
Treasure maps consist of:
Low Map (drops from level 101+ monsters)
Medium Map (25% chance dropping from Low Map treasure chest)
High Map (33% chance dropping from Medium Map treasure chest)
Chaos Map (33% chance dropping from High Map treasure chest)
Insane Map (33% chance dropping from Chaos Map treasure chest)
Alternatively, you can get the treasure maps through:
- Item exchange (e.g. 5 Extreme Stones = 1 Medium Map)
- Dungeon bosses
- Mini Dungeon bosses