As promised in my last post, here are top 3 of my favourite locations on the web for public domain materials:
1) Internet Archive
The front page states that the Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more. This is the site I usually start all my research with but do be mindful that not everything on the internet archive is in the public domain. This is clearly stated in the licensing information of each material you browse through.
2) Project Gutenberg
There is a wealth of eBooks here as the site's mission is "To encourage the creation and distribution of eBooks". The last count was over 50,000 of them. The materials are digitized and proof read by volunteers, so a donation is recommended although not mandatory.
3) WikiMedia Commons
The site states that the page is a collection of 37,281,473 freely usable media files to which anyone can contribute. I usually source graphic images from this site to be added to an ebook or project that I am working on.
The above are only a snippet of the wealth of online resources available that shares public domain materials. There are many others which I will share in due course.
Lastly, do make sure you do your research before releasing or remixing the above mentioned material to sell on as your own. This is due to the reason that copyright laws differ from one country to another. Most of the sites above are based on the common law in the US.
Have fun and do share the products you have created and if there are any other sources that can help others in creating their own product using public domain materials.