That splined shaft is a common style used with other control knobs. Your nearby Radio Shack may have some repair options for you, though not identical to the original. Bring the old parts with you if they give a sense of the size & pattern of the splines. If not RS, any well-equipped electronics parts house should have something that works - or if you see a pile of old (OLD) radios somewhere, start pulling off knobs. A setscrew type would also work - the screw clamps on to the shaft.