One way is to contact the Department directly. Ask for the Media Spokesperson's Office. Then ask if they keep yearbooks. A lot of Departments do. There is usually someone in that bureau that keeps an archive pretty much up to date.
You may also want to contact the Training Academy for the particular police department. They also have some records and usually VERY interesting archival photo's of graduating classes. San Francisco Police Department, for example lines their walls with vintage academy class graduates.
Good luck with your research!