To build on what onebuck said . . .
In order for freedoms 1 and 3 (the freedom to make changes and the freedom to publish the changed versions) to be meaningful, you must have access to the source code of the program. Therefore, accessibility of source code is a necessary condition for free software. Obfuscated “source code” is not real source code and does not count as source code.
All distros that use GPL code must make the source code available in some fashion.
Slackware includes it in the DVD/CD *.iso files. (You can also find it on the website if you nose about for a bit.)
Other distros make it available from their websites or through other means. For example, here's Ubuntu's article about kernel sources (short-form for "source code") and how to get them. https://wiki.ubuntu.com/Kernel/SourceCode
If you are looking at, say, Slackware CDs five and six, which contain sources, you do not need them for the install. They are part of compliance with the GPL. It's an old school means of making sources available, and Slackware is proudly old-school.