Great new source of information about chant manuscripts and also source of chant scores is available online on the site Gregofacsimil. Website is in French language. If you don’t understand French at all, you can try automated translations: English, German. On the page you can find introduction to problematic of restitution of chant melodies, some of its results – especially responsories of selected matins (night part of officium) and also whole matins for some feasts (I haven’t seen scores of matins anywhere else on the net).
Especially useful is large list of sites whith digitalized chant manuscripts. In the list is mentioned type of codex, source, type of notation and other information.