How do you find royalty free music?

Hello travelers,

I've recently bought a DJI Lito 1 mini drone to take with me while traveling so I will be making some nice videos hopefully.

As the drone itself doesn't record audio, having some background music is essential. And if I am going to post it on YouTube it must be royalty free. Any advice where to get some royalty free music, especially for longer videos (10-20 minutes) ?