I believe those are supplied by the MPAA. It cost several thousand dollars for the right to display that. They have to rate your movie, you don't. I'm sure if you do a google search you can find one to put on there but it is not legitimate.
Those ratings are trademarked by the MPAA. You cannot use the G, PG, PG-13, R, or NC-17 ratings unless you have actually submitted your film to the MPAA for its review & rating process.