A SlackBuild (out of which a package is usually built) is available at
http://slackbuilds.org/repository/13.1/system/laptop-mode-tools/. It is maintained by V'yacheslav Stetskevych. The tool "sbopkg" (http://sbopkg.org) is usually used to semi-automate installation from slackbuilds.org.
(If the SlackBuild does not work for you, you may try the following older instructions to install Laptop Mode Tools:
/etc/rc.d/init.d/laptop-mode restart
to your rc.local.
Note that the line is "restart" instead of "start" because Slackware doesn't completely clear /var/run on bootup. The "restart" is needed to make laptop mode tools initialize itself correctly on bootup.)