Where are css injector files stored on the server?
CSS Injector stores it's files in the files directory under: 

'public://css_injector/css_injector_' . $crid . '.css';


as defined in _css_injector_rule_uri

This often maps to the following folder:

sites/default/files/css_injector

See also: css_injector_init for how the files are added by that module.