Try royaltee free website like AudioJungle.

Or copyright-free songs.

Or collaborating with a music for games student as someone suggested.

Or the unity asset store (music category): https://www.assetstore.unity3d.com/en/#!/search/page=1/sortby=popularity/query=category:78

Yeah if game is really good , you could try to reach some new up and comer musicians , if you use they songs its a win-win situanion , just dont forget to highlight them - like put them in the credits , make maybe a music player which shows which song is playing, and put some credit when you advertising your game to advertise musician, it will help to get some fresh tracks, if conditions you propose them is right.

Google. lol. And use it to not just find free non-copyrighted music, but composers who are willing to help you on a budget. I once asked for something for a trailer I was doing, and like 20 people offered free work. It's the internet, there's always someone.