The reason there are no aftermarket cams is the VVEL ... there is an interesting article on a competitor forum (search Mike Kojima+Nissan VVEL) in google you should find it. Wikipedia also has some interesting material.
You can also find a heap more referecens via google, but the long and short of the matter is that the intake cam is continuously variable in timing and lift, and the exhaust cams profile is matched to it, so your regular camshaft guy is not going to be able to assist. If you want an upspec'd engine, then purchase a 350DE block and heads and install a strocker kit with your chosen cams. This is what Nissan have done in GT4 competition in Europe (see
http://www.rjnmotorsport.com ) and in the all Japan GT series with the NISMO 350Z as there are no user or dealer-serviceable parts in the VQ37 VVEL head.
It is worth noting that the throttle butterfly in the throttle body spends "most" of its time fully open as the air-supply at low throttle openings in actually implemented by the VVEL controller reducing inlet lift to <1mm ..... this is one complex engine and it will be a while before the after market cracks the controller logic and the mechanical setup of the VVEl system to come up with upgrades.
If you check the Mike Kojima related URL, you;ll find a video clip and several (5 I think) pages of additional text which dicusses the complexities.
I think you will then appreciate what the Nissan engineers have done.