The copyright law does not define public domain, but does explain the conditions necessary for copyright protection, the types of works that are eligible for protection, the rights of copyright holders and the exceptions to these exclusive rights.
If a work is within the public domain, there are no copyright ownership issues associated it.
This is because the work was either never eligible for copyright protection OR, the term of copyright protection has expired.
Therefore, anyone may reproduce, redistribute, or adapt the work and there is no longer a need to seek permission for its use.
Due to the extension of the terms of copyright by Congress, It will be 2019 before anything else enters the public domain.
Here are a couple of handy tools to help determine what is in public domain in the U.S. as of this year-
There are a number of sources for materials that are in the Public Domain - sources that can be used for free and without permission - below are just a few of the places where you can find such materials:
The Commons on Flickr (Photographs)
Hathi Trust Digital Library
Wikipedia List of Free Image Sites