I´d just get a generic one that can be threaded into the filter thread. Most photo dealers have "junk drawers" with old lens shades, filter cases & c, and they´re gold mines on this kind of occasions.
What´s your camera, btw? If it has an APS-sized sensor, you should try to find a shade for something longer than 50; one for an 85 would probably do just nice. With digital, it is easy to check for vignetting right in the shop.