Purl Strings by Minnie and Purl.
The "Sweater Plus" pack of try on tubing includes 3 silicone cords. 1pc is 100" long for the sweater body, and 2 pcs are 40" long for arms or child sweaters.

This handy tool allows you slide your working stitches onto a long  flexible cord whilst mid-progress and try on your sweater for a fit-check. To use, simply insert the tip of your needle into the opening inside the cord end, slide your work onto the cord, and now you can check for fit as your project progresses.