Making friendship bracelets, threads are the main item. There are many types of thread and many points to look out for. For example, the texture, the thickness, the colours etc. I prefer to use embroidery floss for making friendship bracelets especially DMC embroidery floss. DMC Cotton Embroidery Floss is a soft thread made from 100% long staple Egyptian cotton. It is comprised of 6 size 25 easily separated strands, allow you to adjust the thickness of your stitching by using a different number of strands. Colours are 100% colorfast and fade resistant. DMC Cotton Embroidery Floss is available in 454 solid colors, 18 variegated colors. If you wants your friendship bracelet in different thickness, you can add more strands to it.
There are many other kinds of strings like, craft thread, yarn, crochet thread, cord, leather string, sew thread etc. There all have different textures and thickness. Up to different preferences.
You can get more info from the following link: