Where do you find music that you can get permission to use? For us, the best way right now is to find a friendly musician to work with. We’ve been fortunate enough to have worked with our longtime friends at AudioBody.
There are also music sites that will provide you with music that you can use online, either for free or for a small fee. We’ve had people recommend:
Incompetech has a royalty-free music library, with a suggested donation of $5.00. Your video has to acknowledge where the music came from (attribution required, Creative Commons license), although licenses with no attribution required are available for a fee.
Audionautix similarly has a royalty-free music library with attribution required and Creative Commons licenses.