The program can only be downloaded from the the novocraft website

This is the most direct way:
1) browse to http://www.novocraft.com/support/download/
2) click on the plus-sign next to or on the boldline with the version
as given in the novocraft.info or novocraft.Slackbuild ($VERSION; this will be relatively close to the top)
3) click on the 'Novocraft Programs' for X86-64 Linux 3.1 Kernel
(it will link to http://www.novocraft.com/support/download/download.php?filename=V{$VERSION}/novocraftV{$VERSION}.Linux3.10.0.tar.gz)
download the program to your computer and
move it to the local repository of your Slackbuilding program (e.g. /var/cache/sbopkg/ )