Location of Keyboard Icons?
The flag icons are located in the file /System/Library/Keyboard Layouts/AppleKeyboardLayouts.bundle/Contents/Resources/AppleKeyboardLayouts-L.dat. The .dat file can be opened/read with the GraphicConverter app.

The .dat file is a flag "database". To access the single flags in GraphicConverter use the pages drop-down menu at the bottom right of the main window.



Several input method icons (e.g. PNGs, TIFFs) can be found at /System/Library/Input Methods/Input_method.app/Contents/Resources/... with Input_method.app one of the various IM.apps in /System/Library/Input Methods.

The Custom Input icon may be this one: /System/Library/Input Methods/PluginIM.app/Contents/Resources/Custom.tiff.