Magnetron

Magnetron is a treasure.

Source
Magnetrons can be obtained as an enemy drop from the following creatures:
 * MA Hoplomachus - 5% chance

It can also be obtained as a collectible throughout the world:
 * Gralea

Quests
Magnetrons are needed for the quest A Better Noiseblaster to upgrade from a Noiseblaster to a Noiseblaster Plus.