Cyberhymnal.org has the score in .nwc format, and it also has a midi file (which can become a score, given the right software).
Hymnary.org seems to have the score without words and some midi files in its media, but also lists tons of hymnals that have that song. I don't know about the others, but the 1892 US Episcopal hymnal (The Hymnal) is surely public domain by now, but the version online doesn't seem to have scores. The University Hymn Book has the song lyrics next to the score, not as part of it. And so on.
Melody-only seems to be harder to find than all four parts.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.