Hemp Fibers can be harvested from Hemp plants, commonly found on all Maps in Experimental Rust. They can be harvested by targeting the Hemp plant and pressing the Use key(default "E"). A harvested plant yields 10 Cloth and 1 Hemp Seed, which can be then used to cultivate new plants in the future. Naturally grown Hemp plants can be usually found in forests or near them and in most deserts.
Holding down the Use key will give you the option of taking a cutting from a plant, yielding a Hemp Clone to plant where you choose.