|An organ score from the Nova Organi Harmonia|
Recently, a user of the CMAA Forum began creating a tutorial on the methods of providing organ accompaniment to gregorian chant which many have found useful. His first video can be found below, and the rest can be found here. If this interests you, make sure to subscribe, so that you can continue to receive his new videos as they are released.
While there are collections of scores for this purpose, it is ultimately much more useful and flexible if an organist can learn the methods for improvising accompaniment, which this series attempts to teach.