Well Water, Pure Water, Burning Oil, Holy Sap, Sports Drink, Cooked Bile

[Erroneous]

If you are looking for these items, they exist in dungeons only. Particularly breakable objects like skeletons, pots, crates, eggs, etc... Each item corresponds to a particular element type. And the purpose of these items are really essential when it comes NPC trades.

Well Water (冷たい井戸水)	Temporary increase in target's Earth Element.	Breakable Objects in Earth Type Dungeons
Pure Water (澄んだ深層水)	Temporary increase in target's Water Element.	Breakable Objects in Water Type Dungeons
Burning Oil (焼けつく獣油)	Temporary increase in target's Fire Element.	Breakable Objects in Fire Type Dungeons
Holy Sap (聖なる樹液)	Temporary increase in target's Wood Element.	Breakable Objects in Wood Type Dungeons
Sports Drink (イオン電解水)	Temporary increase in target's Thunder Element.	Breakable Objects in Thunder Type Dungeons
Cooked Bile (煮つめた胆汁)	Temporary increase in target's Darkness Element.	Breakable Objects in Darkness Type Dungeons

The best strategy is to find a room in a dungeon that has a lot of breakable objects, and just spam entering/leaving that room to get large quantities of said item.