Obtaining certain upgrade materials in NieR: Automata proves more challenging than others. Many are enemy drops, but some only appear as random spawns in the game world. These naturally occurring items vary, introducing an element of chance into farming them.
Filler Metal is one such material, requiring overworld exploration for early acquisition. However, a lengthy search is anticipated. Later in the game, purchasing Filler Metal becomes an option, albeit at a high cost. This might be the easier approach if you have sufficient Gil.
Filler Metal is a rare drop from item spawns deep within the Factory. Its location changes with each playthrough, and it has the lowest spawn rate compared to other items. After revisiting the factory during the main storyline, you unlock the Factory: Hangar access point, enabling fast travel. This is the ideal starting point for exploring the factory, providing significant progress.
Depending on your story progression, you might need to unlock the Factory: Hangar access point again.
While increased movement speed aids farming, reliable Filler Metal farming remains impossible. Your best strategy is to thoroughly explore the factory, collecting all naturally spawning items. However, purchasing remains the most efficient method for acquiring large quantities.
The only vendor selling Filler Metal is the Shopkeeper Machine in the Amusement Park, but only after completing one of the game's final endings (requiring all three playthroughs). After completing the game, accessing this Shopkeeper via Chapter Select will reveal Filler Metal in its inventory, priced at 11,250 G each.
Despite the high cost, purchasing is far more reliable than repeatedly traversing the Factory. The Pod upgrades requiring Filler Metal are crucial for game completion, as enemies scale to high levels.