The CGP is the finding tool for federal publications that includes descriptive records for historical and current publications and provides direct links to those that are available online. Users can search by authoring agency, title, subject, and general key word, or click on "Advanced Search" for more options.
A directory listing of U.S. Government publication digitization efforts. The project underwent a systematic sunset in 2018. The above link provides information on where to find archived site contents as well as current initiatives from the GPO.