Call the department and ask for him by what you have and say you forget his last name
look up the department online and search staff page
Facebook his last name , hope he has a profile pic
do you have his badge number? are you trying to make a complaint?

First thing i tried
Honestly if you really slick about it you can do it through a phone call, have a girl call up and ask from officer "Thomas mcreary" , obviously first name is going to be wrong BUT there going to say we don't have a Thomas mcreary but we have a ______ mcreary
or have the girl call up and say she callin from the " law offices of XYZ" and need to speak with ^^^ use my idea from above

what'd he do to you?
Ask the mods to transfer this to Just Buggin Out in case the cop search his name and find this thread.