check slackware mirror first it is in the file /etc/slackpkg/mirrors
Different methods to install slack packages are
It is a command-line and dialog-based tool to synchronize with the SlackBuilds.org ("SBo") repository, a collection of third-party SlackBuild
scripts to build Slackware packages.
sbotools is a set of perl scripts providing a ports-like automation interface to slackbuilds.org.
slapt-get is another method
Currently the two most commonly used formats are tgz and txz. tgz is package compressed using gzip and txz is package compressed using xz.
txz is the current standard for official packages because of the increase compression rate
pkgtools contains the following executable applications, refer to the man pages on your system for more detailed information including options and syntax.
installpkg,removepkg,upgradepkg
slackpkg is to be used to installed official slackware packages from the official Slackware servers and to manage updates and upgrades,