A legal prescription-only one would be phenermine. Good luck getting it. It's a crazy powerful stimulant with abuse potential. Not many doctors will prescribe it.
Any other diet pills would be mostly caffeine as an active ingredient that helps. Just drink coffee instead. You'll loose as much weight. Mostly water weight from the diuretic properties of caffeine, but you will also burn more calories at rest.
Overall though, get a low calorie, but balanced diet, and exercise. Also, studies have shown exercise to be cumulative through the day, so take the stairs. Mini workouts tend have the same effect as a full one totaling the same amount. At least, in terms of caloric output they do.