Acquiring upgrade materials in NieR: Automata presents varying degrees of difficulty. Many drop from defeated enemies, but some, like Filler Metal, are found in randomly generated overworld locations. This introduces an element of chance to farming these items.
Early in the game, Filler Metal requires exploration of the Factory's depths. Later, purchasing it becomes an option, albeit at a higher cost.
Filler Metal is a rare item found in specific spawn points within the Factory. These locations are unpredictable and change with each playthrough. Filler Metal has the lowest spawn rate compared to other items. Unlocking the "Factory: Hanger" access point allows for faster travel and efficient searching. Note that you might need to unlock this access point again depending on your story progress.
While increased movement speed can slightly improve farming efficiency, reliably farming Filler Metal remains challenging. The most effective strategy is to thoroughly explore the Factory, collecting all naturally spawning items. However, purchasing remains the most straightforward method for acquiring a substantial quantity.
Filler Metal is exclusively sold by the Shopkeeper Machine in the Amusement Park, but only after completing one of the game's final endings (requiring all three playthroughs). Accessing this Shopkeeper via Chapter Select after completing the game will reveal Filler Metal in its inventory, priced at 11,250 G per unit.
Despite the high cost, purchasing is a more reliable method than repeatedly scouring the Factory. The Pod upgrades requiring Filler Metal are crucial for completing the game, as enemy levels can otherwise become insurmountable.