There are a number of options available in so far as aftermarket vented bonnets are concerned. Nismo, Seibon and Knight Racer come to mind for instance. But I don't know of any manufacturers that produce just a vent/scoop.
If you have a factory carbon bonnet with the Naca duct, then I would advise you not to even consider butchering this to accommodate a cheap vent/scoop. Rather source a regular R34 GTR bonnet and then cut that to accommodate such a vent/scoop if you so wish.