If you mean the 3/16" plates similar to what's welded onto the spreader bar proper, American Stamping in Mississippi has them.

I got four of them with my re-pro Deuce rails.

Muffler shops carry two-bolt block off plates that - if you can find the right one . . . which shouldn't be difficult - bolt right on.
Saw a set of finned aluminum ones on the rear horns of a chopped five window highboy Deuce once, looked good.