Purl Strings by Minnie & Purl. The Sweater Plus Pack of "try on" tubing includes 3 silicone cords. 1 piece is 2.5 meters long for the sweater body and 2 pieces are 1 meter long for arms or child sweaters.
To use simply insert your needle into the cord opening on both sides of the cord. Move your work onto the cord and now you can try on for fit as your work progresses.