Something may have passed me by, but I'm pretty certain my Mum never bought me gloves with string already attached. I suggest you stop looking and just buy a pair of gloves, a length of ribbon, tape, string or whatever you prefer, and get out the needle and thread. Apart from anything else you can get the length exactly right that way.
I'm often making strings for my grandchildren's new gloves. One easy way if you have a ball of wool is to unwind 4 times as much wool as the length you need to go across the back and down both arms. Next fold it in two. Tie a knot to hold the ends together and slip the knot end round the door handle. Now begin to twist the whole length until it is so tight with twists that you can hardly do any more. Next stage requires care ... hold both end of the wool and gently slip the knotted end off the handle. Keep both ends in one hand and shake hard downwards. You will find that the wool has doubled up on itself and is really strong. It is now ready to be sewn into the cuff of each glove.
I'm looking to make a patent for this it is so foolproof.