Can someone please show me where can i find the source code for the application launcher plasmoid? I am unable to find it on KDE base. Do I have to download entire source for KDE ?
There are many
There are many application launchers for the KDE:
KDE Userbase: http://userbase.kde.org/Plasma_application_launchers
The default application launcher - Kickoff source: https://projects.kde.org/projects/kde/kde-workspace/repository
Since it's been a while:
Kickoff remains the main KDE app launcher. You can find its git repo here.