Crank window runner piece

On a crank window, there's an arm that comes from the frame and attaches to a runner at the bottom of the window.  The arm has a round nub on it and the runner has little square piece with a round hole that the arm nub fits into and then that slides along the runner.  One of my windows has lost that little square piece so the arm has nothing to attach to on the bottom of the window.  I'm hoping to just buy that little piece rather than an entire repair kit just for that one piece.