Music published in or before 1922, is in the Public Domain in the United States.
CPDL is a free sheet music website with an emphasis in choral music. It also contains information about composers and can be a useful resource when you start a search. CPDL has a management team but also relies heavily on volunteers to make additions and edits to pages. As with any wiki resource, you'll want to consult additional resources for accuracy. To find out more about the Choral Public Domain Library, search the ChoralWiki About page.