Most tree barks can be pounded then separated from the fibers. The best are basswood , elm (if you can find somethat's not blighted), walnut,maple, cherry, aspen, ceder.
For fibrorours plants, dogbane, milkweed, velvet leaf, nettle, eve-ening primrose,catail,rushes, saghbrush,stinging nettles,velvet leaf,and fireweed.
The above is from "Tom Browns Field Guide to Widerness Survival"
Have tried some (of the above), but the plants are not all here (Northeast USA). Comments from other users?
Thanks in advance!