This site propose you to get the music you need ! Royalty free means you have to pay only one time, and use lifetime... More info here !
It's very simple : Choose your category, listen, and if something fit your needs click on the "get it" Arrow Icon. You will go on my partners site (AudioJungle or PremiumBeat), choose a version and/or a licence and it's done ! (Track prices are depending on the version and the licence, but it's from 10 to 50$ for common use)