"The Copyright Office Catalog contains approximately 20 million records for works registered and documents recorded with the Copyright Office since 1978.
Included in one integrated database are registrations and preregistrations for books, music, films, sound recordings, maps, software, photographs, art, multimedia, periodicals, magazines, journals, newspapers, [...]"
Orphan works are those works in which the owner of a copyright cannot be established. Orphan works are still under copyright.
Testimony, reports, and legislation from the U.S. Copyright Office can be found on this site.
There are three major performing rights organizations in the U.S.: ASCAP (The American Society of Composers, Authors, and Publishers), BMI (Broadcast Music, Inc.), and SESAC. All license the public performances of their members' works. Each maintains a database which can be searched.
Music Publishers Association of the United States (MPA) maintains a directory of publishers and imprints.
Many music publishers' websites contain links to forms to request permission to copy scores in their catalogs. Here are a few: