Aluminum

Category:Resources
Used commonly in the construction of low-durability modules due to its relative abundance and light weight.
Used in: Practically all tier-I weapons, armor, propulsion, etc. modules ; manufacture of Fighter Drones 
(spoilers!)
 
Common sources: Harvesting of resource drops from broken-up asteroids, bonuses from surveying Anomalies, purchases from Trade factions on the cheap (e.g. Trade Guild, Goryr, Wiskamug)
Remarks: If you build up a reserve of 1000-2000 Aluminum, you should have enough to outfit the largest ship with low-durability modules, or resell your excess to the Island Traders.