Hi Sadie,
I have been using yucca leaf fibers with good success, because finding sinew long enough is often a problem. If you have access to a fresh deer, you can try chasing the leg sinews up into the hams. I have gotten much longer sinews that way, but its hard to do.
I used homemade hide glue to secure them. Josh says If you use them green they will shrink and actually loosen up, rather than tighten up, so dry the fibers out a few days after you seperate them from the yucca leaf, and before you apply them to the arrow.