Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
nvi can be obtained from http://www.bostic.com/vi . This is a web page from which you can download the current version, and also ask to be added to a mailing list that is notified about new versions of nvi and/or new features.
The source code for nvi is freely distributable. The licensing terms are described in the LICENSE file in the distribution, and they permit distribution in source and binary form.