Software Sources GUI in Kubuntu 18.04

It is already installed but is not seen by the launcher by default. In order to make it visible, follow the other answer posted by DK Bose.

It can be launched with the command

sudo -H software-properties-kde

as suggested in a comment by @DKBose

Or, through the Kubuntu's Software -Center (Discover), clicking Settings and the upper-right button

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After having installed LXQT desktop (on top of Kubuntu), what I prefer now is to use the lxqt-sudo tool in the Exec line of the .desktop file /usr/share/applications/software-properties-kde.desktop.

Exec=lxqt-sudo software-properties-kde

which shows the window with Qt-GUI (the Qt widget style of KDE/LXQT set under the LXQT settings) instead of GTK.

It seems that lxqt-sudo has that effect only in LXQT, and not in KDE/Plasma.

The .desktop file for Software Sources is /usr/share/applications/software-properties-kde.desktop in Kubuntu 18.04 but if you open the file with a text editor, you'll see a line with:

NoDisplay=true

One can comment out that line (and do so each time the package is updated). Alternatively, one could copy over the .desktop file over to ~/.local/share/applications and make the change in the copied file.

Whichever way is chosen, Software Sources can be seen in the launcher under System:

Or, it can be accessed via krunner:

Either way, when launched, you'll be prompted for your user password.

To find a clue as to why this application's .desktop file has NoDisplay by default, I ran apt-get changelog python3-software-properties > sof-pro-kde.txt. In there, way back in Karmic Koala, there is this:

software-properties (0.75.4) karmic; urgency=low

  * Suffix software-properties.desktop with -gtk.desktop
  * Create a copy of that file with suffix -kde.desktop
    + Change to KDE icon and exec
    + Add NoDisplay=true, the desktop file is only necessary for kdesudo to 
      have it show a proper name and icon which does not require it to show
      up anywhere