California Car Cover still has OEM covers for Sterlings. That link will bring you to the fabric selection page. California Car Cover has changed their selection menu, so you will have to chose your fabric, then physically call and tell them you want the Sterling GT cover, pattern #2075.
Long lead time? Like what, 2 weeks? If you need it right now, see if a local FLAPS has a generic C3 Corvette cover in stock. It'll be a little large, but it'll fit the proportions.