There are several brands. We carry Stretch Magic. Elastic string comes in different sizes. For most glass beads - .7mm size. For larger hole beads - 1mm size Tie a square knot.
Some people put a drop of glue on the knot.
If you are using glue, use a glue that dries like rubber, such as rubber cement or an epoxy like E6000. When the wearer slips the bracelet over their arm, the hard-bond will crack like a broken bottle, and easily saw through the elastic string with wear. Here you slip either end of the finished bracelet through a crimp bead, and crush the bead with a pliers.