The source code for validns is available under the 2-clause BSD license from github:
Versioned tarball releases of the code can be found here
Packages of validns by Paul Wouters are available for Fedora, RHEL and Centos. Simply
yum install validns.
There is a FreeBSD port of validns by Hirohisa Yamaguchi, which can be installed, for example, by typing
Packages for other platforms will hopefully follow shortly.