Gossamer Floss and Elonga Cord

Rings & Things offers 2 styles of super-stretchy, shimmery multi-strand cord:

• Gossamer Floss is a five-ply, squarish 0.5mm-diameter cord. Double it for extra strength. It knots well, and is excellent for super-stretchy bracelets.
• Elonga Cord multi-strand stretchy cord is available in 3 diameters: 0.3mm, 0.7mm and 1.0mm. All 3 are excellent for bracelets.

String beads directly onto the cord, or use the Igolochkoy needle/threader or Beadalon's Elastic Cord Needle.

You might also be interested in Stretch Magic. It's a bit thicker and stronger, but doesn't stretch quite as much.