Though dropped infrequently, it can be obtained as a reward from Dark Sector defense missions.
There is a relatively easy way to farm this on the Kuva Fortress tile set. In many end rooms, there are Latcher Turrets that will deploy 2 waves of 8 Latchers when your warframe is detected by the scanners. An AoE weapon such as IgnisIgnis or a weapon that chains like AtomosAtomos can make quick work of the Latchers and you can repeat this infinitely. The turret will retreat into the ground after deploying its second wave, you can spawn it again by being scanned another time.