Scrap Metal

Scrap Metal is a Common-tier crafting material with two documented sources: the boss Argus and Monkey Hollows. That gives it an unusual dual supply line - a scheduled boss kill for burst income, and a regular mob type you can farm on demand between spawns.

Its documented crafting use is the Energy Cell recipe, which requires 2 Scrap Metal. That low count plus the Common rarity keeps it from ever being a serious bottleneck, but it does mean Energy Cell crafters should keep a small standing stock rather than special-tripping for it.

As a farming note, Monkey Hollows are the practical route for most players - regular mobs respawn on your schedule, while Argus is a boss encounter with its own timer (and better reasons to fight it than scrap). Pick up Scrap Metal as a side effect of whatever else you are doing in Monkey territory and the Energy Cell recipe will never be waiting on you.