In the Abandoned Templed (Swamp map), there's one larger room with a TON of the lizard monsters. One of the chests in that room has it. To get to that room, you need to fall through the floor, should be the first of the three halls with broken floors at the end.
I've been looking all over the swamp map and can't find the abandoned temple - what am I missing?
It's there -- fairly sizable swamp, but if you go down the main wooden path at the start, (it should be direction screen down), go player-left (so joystick right), and it'll be at the far middle. There's a lot of worms around the entrance, more than other places, and a light in front of the building.