You could commission an artist, use an AI generated art piece, makes one yourself, or use a premade creative commons art piece. Depending on what your price range is I would recommend an artist either through reddit or through one of the art commission sites. I don't remember what the AI is called that generates art but someone here can tell you. If you are good at drawing then have at it but if you're like me and turn everything you draw into accidental threats to humanity then don't. Creative commons art pieces are super easy to find on the internet. Just look up creative commons anime (enter descriptions here) and boom, free art.