Media items can be found easily from the Internet, it doesn't mean it is legal to use it without permission. Beware of the videos/images may have been uploaded to a website without the copyright owner's permission, this is an 'infringing content' and should not be re-used.
Find and use media items which are copyright-free or released under a Creative Commons License that allows non-commercial derivative use.
Or, find and use material that you have paid for.
"This library contains digital movies uploaded by Archive users which range from classic full-length films, to daily alternative news broadcasts, to cartoons and concerts. Many of these videos are available for free download"
Browse the Creative Commons licensed videos on Vimeo. Learn about what you can and can't do with other people’s videos on Vimeo to help you share, rework and reuse legally.
If you are going to limit the use YouTube video with Creative Commons license, please:
Click "Tools" > Selects"Usage rights" > Select license
"Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license".
"This library contains recordings ranging from alternative news programming, to Grateful Dead concerts, to Old Time Radio shows, to book and poetry readings, to original music uploaded by our users. Many of these audios and MP3s are available for free download"
"Freesound is a collaborative database of Creative Commons Licensed sounds. Browse, download and share sounds"
There are some sources of "stock music" sites that sell music, but often not "famous" artists. You can search by genre or style of music and then check on the price of the licenses (prices seem to range from USD 5 to higher).
In Hong Kong there are a number of licensing group you can try; but they are not instantaneous. You may need a few weeks to settle the licensing.