KOrganizer can be obtained from a number of places as source code or as binary file. It is included in the kdepim package of KDE, the solution for Personal Information Management.
Generic distribution as source code from the main KDE FTP site:
Other distributions (rpm etc) and KDE FTP mirror sites:
All major Linux distributions include pre-compiled packages of KOrganizer as part of KDE.
You may also always obtain the absolute latest version right out of the KDE SVN tree. See instructions for details how to get and build it.