They are manufactured by Street Scene Equipment and are called Speed Grilles. They currently only come in brushed aluminum (possibly chrome in the future). They are made for the Main grille and the Valence/Bumper.
Main grille: $139.95 plus tax
Main Grille Part # 95077300
Valence grille: $105.00 plus tax
Valence/Bumper Grille Part # 95077320
Web site is
www.streetsceneeq.com
It doesn't yet list it for the Maxima though.
Credit: Pillager