The Latin Mass Society site includes a wide selection of Ordinaries, the Asperges, and a number of other useful chants. Good quality.
Saint Antoine Daniel Gregorian Chant Ordinaries (Kyriale)
Propers for the Schola (EF)
Searching for keywords in proper prayers will often turn up MP3 files of someone singing the chant setting. It is worth searching YouTube, Vimeo and GloriaTV for videos as well. There are a couple of sites which have attempted to produce a complete set at least of Sunday Masses and important feasts, but these projects are far from complete.
Saint René Goupil: excellent quality, covers the whole year of Sundays for Introit, Offertory and Communion.
CantGreg has a longer list of rather poor quality recordings, but this project appears to have ground to a halt (last updated in 2007)
Propers for Priests (EF)
Windsor Latin Mass (from Missouri, USA) is in the process of putting up the written chants necessary for priests: Collect, Epistle, Gospel, Postcommunion, in the different versions (festal and solemn, 'antiquus' and 'ad libitum'). These are beautifully clear sheets with the entire text set to the square notes. Another on-going project.
The Fraternity of St Peter (FSSP), using the sheets created by Windsor Latin Mass, has MP3s of the Epistles and Gospels for Sundays and feasts throughout the year. Another on-going project.
CDs to buy
There are hundreds of commercial CDs which may be useful for particular things. Produced specifically for scholas to practice are the following:
A CD of the most common and useful chants was produced by the late Nick Gale, see the Gregorian Chant site.
There is a complete set of Sundays and a range of Feast days, plus the whole Kyriale, available (at a price) on CD from the Schola Bellarmina of Brussels. The quality is excellent.