Most of Debian and/or it's derivatives uses apt-get to find the packages they wanted to install on their system. This is working as Debian has a very large repositories which contains lots of packages. The same goes with some other big distribution like Mandriva and OpenSUSE. What about Slackware?
Slackware do not have official repositories outside slackware.com (and slackware.osuosl.org for now) which is maintained by Patrick himself. There are however other repositories which contains SlackBuild Script and/or Slackware packages, like LinuxPackages, Slacky.EU and SlackBuilds project.
Tony from Russia has developed a nice system called SlackFind, a Slackware Packages Search System, which is like Slackware's Package Browser, but instead of searching for official packages, SlackFind can find through several repositories and look for SlackBuild or Slackware packages. So far, they have listed about 7 repositories on their system, but i think it's just a matter of time before more and more repositories are added into their system.
Please note that their system only contains the metadata about the repositories and linked them in on each action. The site do not save any packages/SlackBuild script on their own site.