Hi Rick,
Are you referring to the Hessen wappen/front plate?
There are 2 types of Hessen plate/wappen mounting, diagonaly or commonly refer to the N-S, and horizontally or E-W. I think the private purchase is mostly N-S, whereas the EM is E-W. Which one is yours?
Too bad I dont have a Hessen haube, somehow Hessen haube is very sought after ( I am searching for it myself

).
Do you have pics of the haube that is missing the hessen plate?
Try
ageofkingsmilitaria.com
greatwar.com
weitze.net ---> the best, but very expensive
But it seems that hessen wappen plate is not too common.
Btw, is this the Hessen wappen plate that you are refering? Its an older, 1867 type Hessen haube. Too bad, the hessen is not mine.
hope this helps,
Mush
PS: This guy sells very good repros.
http://www.kammerbulle.de/Hauptseite.htm