Most of the video editing programs on the market have some music available. Pinnacle Studio has a pretty broad range of styles and software to make it the same length as your vdeo... a roll-in at the beginning and fade out at the end for whatever length. You can also purchase songs and music online with rights to use it. Search Google for "free music for videos" and a number of options will pop-up.
Terry
YouTube has audio in the Creator studio and you could check free music archive. Just be sure to give credit to the artist if is under a Creative Commons license.