Cactus Flesh is a rare substance in No Man's Sky obtained via Farming Echinocactus plants at your Home Base or from the same plant in the wilds of Barren Planets.
It's primary use is in the construction of Poly Fibre which is used in many high level Blueprints and Gels.
Where To Obtain
Here is a planet where Echinocactus grows that only requires the first 2 Portal Glyphs to dial:
This is a Barren World with Low to Medium Flora and High Fauna.