Drush core files location
The commands are in a folder called 'commands' in the root of the Drush folder. 

If you don't know where that folder is on your machine, running locate drush.php in terminal should help you find it pretty quickly.

In fact, running locate drush/commands should give you a list of all the individual include files related to commands, but it depends if your Drush files live in a folder with that name.