Lots of materials work for nock points...bowstring, thread, butcher's string, floss, tape, even electrician's shrinkwrap so I've heard. One of the most convenient and surprisingly effective materials is regular masking tape. It's cheap, can be cut to any width/length, wraps on securely and quickly, has 'soft' edges to hold but not crimp the arrow nock, lasts quite well, can be removed quickly and can even be 'custom-colored' if so desired(Magic Marker). Very handy for tuning arrows to bows.