Orokin Cell farming is all about luck, there are some location that wield a higher percentage to obtain an Orokin Cell. The only non-RNG way of getting Orokin Cell is to exchange 10 platinum for 1 which is just a rip off at the Marketplace.
There are 4 types of places you attempt to farm these.
I only recommend Saturn Dark Sector if you can only access Saturn. Infested makes it very easy to farm orokin cell/levels but the tileset get quite crowded after a while
Mimas is a survival mission which I only recommend to farm from when your also attempting to farm Crimson Dervish. You should farm this until around 30 minutes or until Bombards quit spawning.
Seimeni is my most favorite Orokin Cell Farming Spot and also item leveling mission. It consists of only infested units which allows me to go pretty far in rounds. Plus it possess very little danger if you bring along a Vauban or stay on an elevated location where the mob cannot reach you.
Other Missions on Saturn and Ceres
Other Missions on Saturn and Ceres can also drop Orokin Cells but these are very unreliable and only help you farm Orokin Cells instead of both Orokin Cells/Experience and Other Rare Mods.
As of Update 13.8, DE has changed T3 Survival so Orokin Cell rewards sets of 3 instead of 1. Very RNG based but if lucky can provide the best results.
Each boss in Warframe has a chance of dropping an Orokin Cell but General Sargas Ruk and Lieutenant Lech Krill have a higher drop chance because they are from planets that drop Orokin Cells.
By my experience Lephantis seems to drop atleast one guaranteed Orokin Cell when you defeat one of its early heads. Lephantis is probably the best chance of you obtain more than one Orokin Cell at a time guaranteed.
Derelict Defense and Survival
The Derelict Endless Defense and Survival has a chance of rewarding an Orokin Cell every 5 waves or 5 minutes. It’s just like the Void but instead of sets of 3, you only receive one so this is probably one of the most unreliable way of obtain Orokin Cells because its just too time consuming and mainly depends on RNG.