Silk thread is the traditional favorite for stringing pearls, although it can also be used to string and knot other beads.

It's sold on cards in 2 meter lengths (about 6.5 feet) with a flexible twisted wire needle already attached, or in larger quantities on spools, in which case you need to buy a needle as well. Because silk stretches and degrades with frequent exposure to body oils, soaps, etc., you may want to consider nylon thread instead, although it doesn't have quite the same drape or sheen as real silk. Just bear in mind that you will eventually have to restring necklaces and bracelets strung with silk.