There are some excellent resources available from the Church Music Association of America (CMAA) to support the singing of Gregorian Chant in parishes. Their resource page is available here.
Two things I’d especially draw your attention to are the “Parish Book of Chant”, a simply excellent collection of chant repertoire suitable for all parishes and the “Communio” collection of communion antiphons. Both of these are available for free download in pdf format, but hard-copy versions at reasonable prices are also available.
There is a lot more available, including reproductions of many of the older chant books. For example, the 1961 English-introduction-and-rubrics version of the liber usualis is available in pdf.
Beyond the resources page, it’s an excellent site for chant junkies to browse and there’s even a forum!
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