Ever heard of Public Domain, Creative Commons, CC0.. ? .. if not.. you should!
Most of the content you find on the web is not for free. You are 'allowed' to watch it. That's it.
But there are generous people, who contribute their stuff for free to the world. You are even allowed to use it commercially.
Here I drop some ressources i stumbled upon:
Free Images/Illustrations: unsplash.com, pixabay.com
Free ClipArt: openclipart.org
Audio/Music: look for public domain stuff
Fonts: fontsquirrel.com -> read this
Sourcecode (look for MIT Licensed stuff): github.com