well , for starters phobos is the only place to get plastids if you havnt unlocked uranus or saturn .
plastids are rare and have a low drop rate ( highest is probably 24 ) farming plastids is just like farming warframe, do a mission where it is said to have lots-o plastids ( most likley survival or defense about 5x )
Lower. Level. Fortuna. Bounties. They'll yield around 200-300 plastids. Yes, even they are a bit difficult to farm for since other things are in the drop tables, but it's the easiest way for players to earn plastids in bulk (that is, players who don't have access to Phobos, Saturn, and Uranus).