There was a guy on here selling alot of parts, Grilles were some of them.
It is a bit of an old post now, you might get lucky with him.
http://www.datsun1000.com/TopicView.asp?TopicID=83

Yahoo Auctions has one for a coupe at the moment. (with headlight rims also).
You will need to use a company to bid on your behalf, or have some one with a postal address within japan to send you the grille.
http://page7.auctions.yahoo.co.jp/jp/auction/g105230455