Where can I find the source image files for the browser checkboxes in Safari?
Download ArtTools. It helps you decode the UI elements file.
Go here: System/Library/PrivateFrameworks/CoreUIFramework/Versions/A/Resources.
Copy the *.bin files to a folder on your desktop.
Copy the *.jar files of ArtTools to the same folder.
Run the following command: java -jar artDecoder.jar SArtFile.bin SArtFile.bin/.
Do this for all the *.bin files. 
Browse through the extracted files and you will find the checkbox there.