This item can only be acquired in the Grineer Shipyard tileset on Ceres, occasionally dropped by Eximus units on death. This resource will not drop from enemies in other tilesets, regardless of Eximus status.
Prosecuters count as Eximus units, thus dropping Cubic Diodes, making Exta a good farming spot.
Kuva Lich missions can spawn Eximus units more often as a result of their higher level, allowing for easier farming.
Seimeni can be a good node to farm if no higher level missions are available, providing 80 Cubic Diodes in one 20 wave set in one test.
The Mobile Defense node Kiste can be good for a quick way to farm due to the high chance of Prosecutors spawning. These Pprosecutors are considered as Eximus units and can drop the resource.