If you search Google for CA traffic court discovery you will find a number of samples. Best bet might be to send the request to the DA and the local police agency that issued the cite. Keep in mind that the police might not be legally obligated to provide Discovery to you, but many will do so anyway. The DA will likely send a letter back saying they don't have anything. But, since the law indicates they are the ones you seek discovery from, they should be in that loop. In some counties they will forward the request to the appropriate agency - if that agency is clear from the Discovery request.