Purl Strings by Minnie and Purl.

The "Sweater Plus" pack of try on tubing includes 3 silicone cords. 1pc is 2.5m long for the sweater body, and 2 pcs are 1.0m long for arms or child sweaters.
To use, simply insert the tip of your needle into the opening inside the cord end on both sides, move your work to the cord, and now you can check for fit as your project progresses. Comes in a metal tin.

Purl Strings are innovative, reusable stitch holders that can be used in place of waste yarn.  The smooth, tube-style cord allows you to simply put the needle tip into the end of the tube & slide stitches onto the cord.  Stitches are safe & then slide easily back onto the needle when you are ready.  The cords are brightly colored so it is easy to see stitches.  Each pack comes with multiple cords of set lengths, but they may be cut to different sizes if desired.