Check out any available books about the agency and study everything you can. Many of the pertinent questions for your story will emerge from your own initial investigation. When you know all you can about the agency, follow up with a phone call to the closest office. Get an appointment to personally interview anyone in the department who is willing to visit. When you meet, be prompt and appear professional.
Cops size people up quickly, and they prefer a conservative appearance.
It's a little like undercover work, you don't want to show up looking like Dennis Rodman, because you, too, are being silently interviewed by your subject. If the officer thinks you're clueless or flaky, they'll probably blown you off. You will probably get helped to the door. A cop arrests people for a living, and doesn't have any idea what you need. Give the officer a thumbnail outline about your completed research.