If you're not concerned about "authenticity" then what I used on my '31 Cadillac brake booster shaft was a bicycle inner tube, clamped both ends with hose clamps. Keeps dirt, water out, seals perfectly. Use a larger inner tube if necessary. Didn't want to pay Steal $135 for something you can't see. I've also used bellows from motorcycle forks, inexpensive and looks just like original.