I would try to find a local welder, or maybe a paid project for the high school metal shop, to fabricate one from heavy gauge sheet steel. The old one can still be used as a pattern, and with a little hammering and plasma cutting, a satisfactory replacement might be possible. Meanwhile let Sears know what you think of their discontinuing a critical part. Chances are the shredder was manufactured for Sears by another company that is no longer around.