You may not like my answer, but I prefer to make radical, start-from-scratch changes, so here goes:
Install Debian and learn about installing packages from the three different branches of Debian (stable, testing, still-in-development), and "pinning" packages. Debian must be the distro with the greatest number of packages available by far, and apt-get is one of the best ways of managing them. The .tar.gz packages used in Slackware, with all their minimalist beauty, may not serve you quite as well.
If you want to stick with Slackware, try some of the other package management systems, e.g. swaret (NB: I am only mentioning it, not recommending it!)
You may also need to edit your configuration files to add further sources to install packages from, which will expand the range of packages available to you. Many developers will make dedicated packages for the main distributions available. A .tar.gz may be available for the software you want, but you probably know that source files are also packaged in this way.
If you have a fast computer, you can use a program called checkinstall to help with the compilation process and generate and install a slackware package at the end of it: usually, "./configure && make && checkinstall" will do the biz.