The osgNV distribution and related packages are available for download at the osgNV project pages on SourceForge.net. You may also want to look at the Prerequisites page to see what third-party libraries and packages you need to download and install in order to make osgNV work.
Just as a reminder, please avoid installing the original nvparse library from nVIDIA because it won't work. You can find a modified, more stable version of the same library on the osgNV project pages. If you are not interested in compiling and using the osgNVParse module then you can safely ignore nvparse and just exclude osgNVParse from build.
If you want to keep your source tree up-to-date with latest development you may update source files from the SVN/CVS server as explained in the SVN Access page. You should first download the client software from the SVN project page, which is mainly used by the new osgNV project after the year 2008.