Jungle Key

 can be obtained from killing the Jungle Key Guardian which has a chance to drop Jungle Keys, or by buying it directly from Odawa for 500 Sludge.

Jungle Key Guardian
The Key Guardian can be found in a room made of granite in the Jungle of the Crystal Hollows and has a chance to drop Jungle Keys. Attacking the Jungle Key Guardian will cause nearby Kalhuiki Tribe Members to attack the player.