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.
nvi builds and runs under UNIX. It also can be built to run under LynxOS 2.4.0, and possibly later versions. It may build and run on other POSIX compliant systems as well, but the documentation does not contain a specific list of known operating systems.
Compiling nvi is straightforward. Retrieve the distribution via ftp. Uncompress and untar it, run the configure program, and then run make.
$gzip -d < nvi.tar.gz | tar -xvpf -
... $cd nvi-1.79; ./configure
... $make
...
nvi should configure and build with no problems. Use make install to install it.
Should you need to report a bug or problem in nvi, the person to contact is Keith Bostic, at bostic@bostic.com.