Music is such a big deal, I’m sure you have watched a movie.
If you are going to submit to festivals or even upload your video to youtube you should own the rights (or at least festival rights) to that selection.
-Find someone to compose and perform a new piece of music. (try colleges)
-Have someone perform a classical piece of music (from a WAY dead composer) ((remember the music might be in public domain but the performance probably isn’t))
-Buy royalty free music, for a set price you may use a piece of music without having to pay performance fees.
The company’s that I listed for Sound Effects also sell music by the track and CD and just search Royalty Free Music and you will find tons of sites (prices will widely vary).