For standard repertoire students become very familiar with www.imslp.org. It is a free online public domain website to download and print music. Great works by Bach, Beethoven, Schubert, and many more are available there.
Want to get better, but you have limited time!? Check out my colleague Andrew Holmes' website. He is a graduate string player from the prestigious New England Conservatory.
Other marching bands expectations and outlines, music websites, and more!
Be inspired! Make friends! And Network.