Naturally grown Hemp plants can be usually found in forests, or near them, and in most deserts. Hemp Fibers can be harvested from Hemp plants; the wild kind are 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 [citation needed as per cloth count and seed], which can be then used to cultivate new plants in the future.
Holding down the Use key on Hemp plants grown from seed will give you the option of taking a cutting from a plant, yielding a Hemp Clone to plant where you choose.