What Are Plastids
Plastids are always dropped as a secondary resource. You will always get less than the other stuff that you collect. Be it Nano spores or even Polymer Bundles. When it comes to Plastids I always go to three different missions on three different planets. They all yield somewhere between 70 and 110 Plastids per five minutes.
You can pick either Dark Sector Survival on Eris and get Nano Spores and Neurodes and Mutugen Samples. Or you can do the Dark Sector Survival - Pescinas on Saturn and get again, Nano Spores, Orokin Cells and Detonite Ampules. You can also do these Submersible Survival - Ophelia on Uranus and get Polymer Bundles, Gallium, Detonite Ampules, Tellerium and the new Excellence mods from the Drekar Grineer. Whilst these three missions are probably the best for farming Plastids, the other endless missions aren't bad at all. You will still get between 50 to 90 Plastids per 5 minutes.
There is one thing that needs to be mentioned when it comes to Plastids. The amount of Plastids you get can actually TRIPLE if you bring Nekros in. The best mission not just for this farming but for farming almost anything is by far the Ophelia Mission on Uranus. You can't go wrong with it if you are in the farming mood.
However, if you have a thing for breaking stuff around to gain rewards, you can just take your Kubrow (if you don't have one get one) and start to hammer the containers on any regular mission. You can get between up to 100 Plastids just by smashing containers. And you also get the satisfaction which we all have over breaking stuff in games for rewards.