gnuplot is available from a variety of sources. The gnuplot home page is http://www.cs.dartmouth.edu/gnuplot_info.html. From this page you can download the software and find some very good documentation, including postscript versions of user's guides and tutorials.
gnuplot is also available on several Linux distribution CDs, including Red Hat, Debian and Slackware.
NetPBM is available via FTP at ftp://ftp.x.org/contrib/utilities/. It is also available on the Red Hat and Debian distributions.