1. Adoremus - the Society for the Renewal of the Sacred Liturgy established in 1995 to promote authentic reform
of the Liturgy of the Roman Rite. Their mission is to "rediscover and restore the beauty, the holiness, the power
of the Church's rich liturgical tradition"

2. CanticaNova Publications (USA) - "Traditional Music for the Contemporary Church"

3. Church Music Association of America - free chant resources are available here:

a) Scroll down the right-side headings to find under Chant Resources:

"Kyriale (Solesmes edition)" containing chants for the Ordinary of the Mass (e.g. Kyrie, Gloria, etc.), and
"Graduale Romanum 1961" containing chants for the Proper of the Mass (e.g., Introit, Offertory, Communion, etc.). 
Use "Index of the 1974 Graduale" with the latter to find chants assigned according to the current liturgical calendar

b) and under Teaching Aids:

"An Idiot's Guide to Square Notes" shows how to read chant notation, and
"Catholic Church Music by Richard Terry (1907)" is a guide to the pronunciation of liturgical Latin

c) Also available are FAQs on sacred music drawing from the teaching of the Second Vatican Council,
Pope John Paul II, and Pope Benedict XVI (right click the above link and save the file to your desktop).

4. GIA Publications (USA) - originally known as the Gregorian Institute of America, GIA now offers "music in all styles for today’s eclectic church"

5. Magnificat Music (UK) - founded by Paul Inwood in 1975, Magnificat Music was the nucleus for the St Thomas More Group of composers. Since 1995, it has been the leading independent publisher of sheet music for Catholic churches in the British Isles. Its publications are used by Anglicans and Catholics across the USA and Canada, as well as from Belgium to New Zealand

6. National Association of Pastoral Musicians (USA) - an organisation of choir directors, musicians, cantors and liturgists who recognise and support the value of musical liturgy

6. Oregon Catholic Press (USA) - the Catholic Truth Society of Oregon was established more than 80 years ago
to provide Catholics and non-Catholics with information about the church and its educational and charitable institutions. In 1980, the newly-named OCP began publishing liturgical music and its programmes are used
in two-thirds of Catholic churchs in the USA today