Resources are items that are used solely for the manufacturing of other items in the Foundry with Blueprints. Other than Orokin Cells, each resource can only be found in specific planet systems. Resources can also be a reward for alert missions as of update 9.3.4. Alert resource awards do not come as a set quantity but will yield far more than a normal drop. This is not the case for rare-class resources.
There are three rarities of resources: Common, Uncommon and Rare.
Common resources can be found in quantities of hundreds at a time. They drop from lockers, breakables, normal enemies and bosses.
Uncommon resources can be found in quantities usually between 10 and 30. They drop from lockers, breakables, normal enemies and bosses. Some of the uncommon Resources are found in higher quantities between 50 and 100, such as Alloy Plate.
Rare resources will usually drop in quantities of 1. They are most often found by defeating bosses, and very rarely from normal enemies or storage lockers/containers. All rare resources can also be found from assassination targets, such as Councilor Vay Hek and the Jackal. As of Update 9 rare resources can be seen in quantities of 2 and even 3 per drop.
It is recommended to farm resources either solo or in a pre-formed group. This will reduce any tensions that might otherwise come up between players who are trying to farm resources and those who do not wish to look for resources.
Break everything. Look for open containers or crates, and do not be shy to break them open, even if your team decides it's not worth it.
When playing a level with a one-way objective with more level behind, explore the entire level, including taking elevators to floors where the objective isn't located to look for more containers. Note, however, that choosing to farm and get left behind may result in a forfeiture of the mission credits, which can only be earned by getting to the extraction point before the timer ends.
Infestation levels tend to spawn more enemies, which means more chances for resources.
Thief's Wit enables the player to spot all breakable crates within range through the mini-map, along with highlighting mods through walls.
Master Thief grants the player a certain chance to open locked lockers (40% at level max).
Codex Scanner can be used to see all containers in current room within the range of 50 meters.
Recommended Farming Locations
These recommendations are based on player opinions and may not be optimal. See discussion comments and individual resource pages for more details.