If you actually like your boss, and see him or her as a true advocate and mentor, finding a gift should be a delight and a breeze because you probably already have a clear idea of his/her interests and hobbies. If you have the misfortune of working for someone you aren't too keen on, thinking about getting them a gift is probably the last thing you want to do. However, you shouldn't leave your boss out. Here's your opportunity to be considerate and to try and repair a strained relationship, or keep things as smooth as possible for the time being.
For the boss, it may be worthwhile to conspire with your fellow co-workers to pitch in and get one perfect gift for your boss. That way, no one will unintentionally stand out like a sore thumb.