(in alphabetical order)
These are some important music publishing companies:
Alfred Publishing Company www.alfred.com
Boosey & Hawkes www.boosey.com
Breitkopf & Haertel www.breitkopf.com
C.F. Peters Corp. www.edition-peters.com
G. Schirmer www.schirmer.com
Hal Leonard Corp. www.halleonard.com
Mel Bay Publications www.melbay.com
Oxford University Press www.oup.com/us
Ricordi www.ricordi.com
Schott Music International www.schott-music.com
Universal Edition www.universaledition.com
If you want a more thorough list for both domestic and foreign publishing companies, visit the Music Publishers Association page (MPA) : www.mpa.org