There are a number who sing the plainsong propers. If you post your general location, I'm sure the members of this forum would be more than happy to direct you to one that may be near you.
I don't think michael.c.x is asking about Gregorian chant -- the article he mentioned begins, "Before there was Gregorian Chant, there was Roman Plain Chant." I highly doubt there are any parishes that use Old Roman chant in the liturgy, except maybe as an occasional one-off art piece.
It's a great sound, but as this NYT reviewer put it, it's on "the fringe of retrievability": that is, these performances are attempts to reconstruct music based on conjectures about the performance practice of the time.
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