As written in a previous post, it’s possible study for the VCP-NV exam using only on-line resources, but the same it’s not really possible for the Network Certified Implementation Expert (VCIX-NV) certification that require really a good practice with the interface and a well understand of all NSX concepts. This exam is available since October 2014.
As usual, the first source of information is the Exam Blueprint (actually in version 1.7, dated 9 December 2014) that give you a lot of information about the exam and also the environment (note that actually there isn’t a simulator or a demo or a mock).
The virtual environment consisting of a management cluster, multiple compute clusters, and related virtual machines. The lab environment is based on vSphere 5.5 and NSX for vSphere 6.0.2.