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.