Plant fibers are made by salvaging grawl necklaces I believe. I have a few of them and am ready to move over to post searing tonight. I need to try the charr stuff past the wall though 1st.
Linen can be crafted from Plant Fibers at the vartious Artisans throughout Tyria, or use an expert salvage kit for a chance to salvage linen directly (instead of plant fibers).