While all the listed locations are good spots for farming Plastids, I was just leveling my Excalibur Prime and went back through some Saturn Missions. I encountered Mimas, a Survival mission. Played it for about 6 minutes, ended with aprox 150 Plastids. Went back for 10 minutes, came out with 331 Plastids. No joke this is the best Plastids farming place I've ever seen. It gets kind of crazy if you frame is like mine, and only 23, but im bringing my Ember back there later. All Infested mobs, and only levels 20-40.
Saturn is a very good place to farm plastids, I have have amazing amounts at Mimas in saturn, I ran it twice. First run i stayed for about 25 minutes and got over 500 plastids, and second run i stayed for 15 minutes and got about 300 plastids, Very good place to farm plastids and highly reccomend it.