The KWWidgets source tree can be retrieved from Kitware's CVS repository using any cvs client:
cvs -d :pserver:firstname.lastname@example.org:/cvsroot/KWWidgets login
simply hit return when asked for password
cvs -d :pserver:email@example.com:/cvsroot/KWWidgets co KWWidgets
If you are behind a firewall that does not allow you to checkout the code, you should still be able to download an archive of the whole repository created each night: KWWidgetsNightlyCVS.tar.gz. Note that the KWWidgets API is available for download as a single offline archive file: KWWidgetsNightlyDocHtml.tar.gz. Both files can be found in the KWWidgets Nightly Repository.
Check the build instructions for more details about compiling the project.
The CVS repository can be browsed online from its ViewCVS interface.
Feel free to check the KWWidgets dashboard too, it reports how successfully KWWidgets compiled and tested on many different platforms and compilers every day. If it is mainly "green", you are good to go. If there is a lot of "red", you may want to check the dashboard in a day or two, and download at this point. You can also browse the dashboard back in time and checkout the CVS repository at that specific point.