Royalty free music is much cheaper than exclusive tracks.
Check out Scirra's Store: https://www.scirra.com/store/royalty-free-music
However, keep in mind that royalty free license means that the seller keeps the rights to sell same tracks to others. So if you don't want your game's music to be used in other people's projects (personally, I don't like it much), then royalty free soundtracks is not for you. On the other hand, hiring a composer for exclusive soundtracks will cost you a lot, especially if you need more than one track.
You can search in Google something like, "royalty free soundtracks". But read carefully about the license agreement because some sellers require buying more expensive license once you have sold 10.000 copies of your game. Just avoid those sites...
So, as you can see, that's the reason why I started this topic. I thought maybe someone would suggest something for indie developers who can't afford hiring expensive composers to get some exclusive tracks.