Friend, i believe that to be grass. Where are you located? If in the eastern US, it is probably a type of grass.
Now if it is good for cordage... I will be cutting some myself this summer:D
I do not think it is flax.
Try some out, let us know!
-Squirrel
As for buying it, try knitting/cloth/craft shops- they often stock linen cloth, unbleached is harder to find. Unweave the cloth(Yes, I really do this. It is hard for me to buy stuff over internet) and untwist the threads, pull the fibers out. I do this with jute-burlap. Also with other cloths... but some fibers are hard to find in cloth.