Works that are in the Public Domain are freely available for you to use outright or to transform into something new. While you do not have to give credit for Public Domain works (according to a 2003 Supreme Court ruling), you probably want to model how not to plagiarize for your students.
There are some types of works that are in the Public Domain:
Where to find Public Domain creations?
Guide to Finding Public Domain Works Online
Public Domain Texts:
Public Domain Images:
Public Domain Audio & Video:
Public Domain Federal Government Resources: