The GABC Transcription Tool will create pdfs of a huge number of chants, according to the melody given in the Graduale Romanum and according to whatever Psalm-tone you like. It also does the lections.
All the basic chant books have been scanned and made available for free download by the Church Music Association of America. There are masses of things on two pages of their website: Music Downloads and Literature for Download. The main ones are listed below. No doubt there will be more as time goes on. They are all pdf files, many of them extremely large.
A number of these can be bought as hard-copy reprints from Lulu.com (see below).
In addition to the Offertoriale edited by Ott, which is available as a dowloadable pdf (see the list above) and as a reprint from Lulu (see below), there is an on-going project to put online newly edited versions of these verses on a German website, Gregor & Taube, alongside a complete Sunday Graduale for the Ordinary Form of the Roman Rite, edited from early manuscripts.
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. An on-going project.
You can now buy a book of these, suitable for use in the liturgy, for all the Sundays of the year: the Canticum Clericorum Romanum. See a review here.
Also available for download is the 1974 edition of the booklet Jubilate Deo, which Pope Paul VI intended as a minimum chant repertoire for parishes. It is available here in various formats.
There are a lot of interesting historical editions of chant books which have been scanned and can be downloaded for free, provided by Corpus Christi Watershed (there is an overlap with the CMAA list above).
The Global Chant Database is 'a searchable database of plainchant melodies and texts included in medieval sources and new editions.'
The Latin Mass Society now has a set of chants for the feasts proper to England and Wales, which are otherwise difficult to find. They also have other resources for chant scholas which are worth looking at.
A splendid online resource for medieval chant is The Sarum Rite, an ongoing project masterminded by Prof. William Renwick: 'The Sarum Rite is a multi-volume performing and scholarly edition of all the surviving liturgy and music of the Sarum Rite, published on-line by the Gregorian Institute of Canada.'
UK sources for newly printed chant books include St Michael's Abbey (Farnbrough) and Cenacle Catholic Books; St Philip's Books of Oxford has some new books, and is a good source for second-hand chant books.
The CMAA's page on Lulu has a lot of useful things, including a reprint of the 1961 Graduale Romanum, which is particularly difficult to find (though some edition of the Graduale is essential if you want to sing ferial days in Lent, which do not appear in the Liber Usualis), in two volumes, hard or soft bound. Lulu is a 'print on demand' company: they don't carry stock, but will print any book given to them in pdf form if it is ordered. Although it is based in America, UK buyers seem to get books at high speed from a UK printer. There are steep discounts for larger quantities.
The Abbey of Solesmes sells a number of their own books directly.
Libreria Editrice Vaticana, the Vatican publishing house, has a growing range of historic reprints as well as books for the 1970 Missal.
The basic book for most Chant purposes is the Liber Usualis. This can be bought from a UK supplier for £79.50 from Cenacle Catholic Books.