Any luck yet?
So others know what you are looking for, this is the diagonal brace just behind the door that goes from the floor sill to the belt rail wood. This part is shared with all '28-29 Coupe models. It is symmetrical, i.e., the same brace is used on both sides unlike '30-31 models that had right and left versions.
Many folks have extra Coupe body parts so it shouldn't be difficult. You can also try Steve or Phil at Bert's Model A in Denver.