An Orokin Cell is a rare component that can be found on Saturn, Ceres and Orokin Derelict. It is usually found in quantities of 1-2.
All bosses (including the Stalker) have a chance to drop an Orokin Cell after being defeated, though General Sargas Ruk and Lieutenant Lech Kril have a better chance to drop it, the former of which are the respective bosses of the planets.