There is a mass of stuff on the internet helpful to Chant singers but it is not all easy to find. It divides into different categories, which have their own pages:
Music: Downloadable Music files and CDs you can buy
Books of Chant music, downloadable or hard-copy
Books about chant, downloadable or hard copy
Groups and individuals who can provide training
The Latin Mass Society website offers a guide to scholas singing at the Extraordinary Form (Traditional Mass), about when to sing and how long you can sing for. See the Guide here. They also have dowloadable booklets of the chants of some national and diocesan feasts celebrated in England and Wales.
Music for the Faithful assisting at Mass, with translations of the Ordinary and propers, are available for many Sundays and feasts here.
An interesting alternative to chant for Mass propers is worth recording:
Propria Dominicalia (SAB)
Cyr de Brant (pseud. of J. Vincent Higginson) Published in 1949
Proprium de Tempore – The Proper of the Mass for Sundays and Holidays Vol. 1 (SATB), and
Commune Sanctorum Missae Votivae Proprium Sanctorum – The Proper of the Mass for Sundays and Holidays Vol. 2 (SATB) Augustus Edmunds Tozer, 1857-1910