If you want a lot of Tri-Lithium fast, the Pluto Dungeon is the way to go—if you're a Cleric. Whether due to bugs or by design, the last two levels of this dungeon don't really work correctly. Getting to them is simple enough—you can follow double ladders all the way down to level 14. Once there, you'll be on a single ladder leading further down. The whole floor is secret doors, and you can find your way back up if you look, but level 15 is a different story.
Level 15 has only one ladder, and it leads further down. Well, in theory—it actually leads to "level 10," but it's a level filled with empty space and fake walls. Once there, the Cleric's SurfaceSpell is your only way out. Because there are no open spaces on level 14, you can't use Up Ladder to return there.