Where are css injector files stored on the server? I'm using this module:

https://www.drupal.org/project/css_injector

In the backent, the files show up at this path:

public://css_injector/css_injector_20.css


But in FTP I look in public.html and I can't find theme

They are not in here either:

/home/vincoixt/public_html/xuzo.com/sites/all/modules/css_injector


So where are they?
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.