Link to Open resources

Some websites when you can found resources in Public Domain or with Creative Commons license:

• Europeana - is Europe's digital cultural heritage platform. over 20 million items – is openly licensed and can be freely reused in various work and learning projects.
• Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more 

For images

• https://pixabay.com/
• https://snappygoat.com/
• https://free-images.com/
• https://www.flickr.com/photos/britishlibrary
• https://www.europeana.eu/portal/en
• https://www.flickr.com/creativecommons/
• https://ccsearch.creativecommons.org/
• https://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources
• https://www.pexels.com/
• https://www.freepik.com/
• https://unsplash.com/
• http://whyopenresearch.org/gallery (cartoons)

For art

• https://images.nga.gov/en/page/openaccess.html
• http://www.openculture.com/2015/03/download-100000-art-images-in-high-resolution-from-the-getty.html
• https://www.pexels.com/search/art/

For audio

• https://freesound.org/
• http://soundbible.com/free-sound-effects-1.html
• https://www.jamendo.com/
• https://soundcloud.com/freemusicforvlogs
• http://freemusicarchive.org/curator/video
• https://archive.org/details/audio
• https://www.youtube.com/audiolibrary/music
• https://creativecommons.org/about/program-areas/arts-culture/arts-culture-resources/legalmusicforvideos/