You can use the search tool on github to find what you are looking for:
https://github.com/kovidgoyal/calibr...utf8=%E2%9C%93
From what I've seen, the concerned files seem to be :
- src\calibre\gui2\library\models.py for the icon displayed in the main calibre view
- src\calibre\gui2\library\delegates.py for the icon displayed when you set the value for the column from the main view
- src\calibre\gui2\custom_column_widgets for the icon displayed when you set the value for the column from the edit metadata view
It's easy enough to change which icon is used, but if you want to add more choices there is probably more work involved.