In order to maximize your farming potential, raise your wanted level to 4 stars.
Where to Farm Toroids
Go inside the target area. Try to get one of the enemies to put down a Reinforcement Beacon. Wait until the beacon hits Alert Level 4. From this point, unlimited enemies will spawn. They will increase in level very quickly, but will eventually hit a level cap that is based on the bounty tier, and stop increasing further.
Vega Toroid: Drops from enemies around/inside the Spaceport. Has a 1% chance to drop from Mite Raknoids. The Exploiter Orb will drop a large number of Mite Raknoids durring her first stage whenever she moves across the battlefield.
Calda Toroid: Drops from enemies around/inside the Enrichment Labs. Has a 20% chance to drop from Scyto Raknoid. Doing all of a Profit Taker Phase 2 bounty but not turning it in will endlessly spawn Corpus dropships. You don't even need to let the beacons hit Alert Level 4 since the drop ships will bring plenty of enemies.
Sola Toroid: Drops from enemies around/inside the Temple of Profit. Has a 20% chance to drop from Kyta Raknoids.