To obtain the seeds, you have to find a wild Plant Species Z plant. It is not harvestable, instead you have to keep standing around next to it. Every 60 seconds it will retract its tentacle and spit out a seed. Some Plant Z Species are bugged when they spit out a seed, where when the seed hits the ground it will disappear after a second or two. A way to fix this is to watch where the seed hits the ground and place a foundation at that location. The seed then hits the foundation and won't fall through the ground or disappear.