Looking For Plainchant (Cantopiano)

How can I find plainsongs (plainchants)? Also, Gregorian Chants, Byzantine Chants... As they don't really have "composers", it's quite difficult to find. I'm interested in both audio and score. If you guys know any good website, pleas tell me^^

I love gregorian chant, and very much like what Durufle and the likes have done with it.

I bought a Roman Missal, and have an Ordinarium with even more melodies. I dont know what exactly you are looking for, but this is a start. Check http://www.sanctamissa.org/en/music/gregorian-chant/ for a lot of free PDFs

it's quite difficult to find. I'm interested in both audio and score.

I'm currently looking for examples of Gregorian chant for my students, and they are actually very easy to find in the internet, both scores, audio and youtube video with scrolling score. The Gregorian is compiled in the Graduale Romanum, the Liber Usualis and a few other books, which have been around for centuries and are available online. You will need to be familiar with the square notation in tetragram, though, but that is quite easy to learn. If you want modern notation, I'm afraid you must do the transcription yourself.

Thanks everybody for the information^^