tonedear.com - This website creates exercises for you to practice a number of ear training elements that come in handy during dictation and music theory in general. You can also use this site to practice short melodic dictations, identifying chord progressions, chord qualities, scales, and intervals. Make these quizzes a part of your daily life for a month and you'll be well on your way to feeling very comfortable with the pitch element of dictation.
teoria.com - This website creates more complicated dictation exercises that include both pitch and rhythm elements. It's far more true to the dictation that we will most often encounter in class.
musictheory.net - This website has tutorials and exercises on a lot of topics that will help you get better at reading music notation. It can also give you some practice with a lot of basic music theory concepts as well.