Braided Elastic 1/8" White - from Dritz
 
This elastic has an improved quality over other brands that can be visually seen and experienced! Your basic traditional elastic lightweight & thinner than woven. Slightly ribbed. Narrows when stretched. Insert into casings or stitch directly to fabric for waistbands leg openings, sleeve edges & necklines. Size 1/8in (3.1mm). 4 yard package.
 
Londa says:  "This is my favorite elastic for pulling tight to shirr seams in a garment.  See my Sensational Shirring pattern HERE and shown in the 2nd image above.