Kelp Sacs are a rare substance in No Man's Sky obtained from the roots of select underwater plants.
These plants are generally only found on Ocean Planets of all hazard types & can be spotted fairly easily via the glow they emit.
It's primary uses are in the construction of Weatherproof Rubber & NipNip.
Where To Obtain
Here is a planet where Kelp Sacs can be found in it's oceans that only requires the first 2 Portal Glyphs to dial:
This is a very pleasant but barren world, it has no negative environment but does take life support at the 2 arrow rate, so taking some gel packs is advised.
Flora is Minimal
Fauna is Non-Existant
Sentinels are scarce
Fantastic World just for farming your Kelp sacs, The Ocean plant that gives the Kelp Sacs can be easily identified via it's glow making night farming an absolute breeze.