Professor of Organ at the Geneva Conservatory until 2001, Lionel Rogg has welcomed students from all over the world. He frequently gives master-classes and is now professor of organ and improvisation at the Royal Academy of Music in London.
He has written an Improvisation Course for organists, published in French and English by Editions Musicales de la Schola Cantorum. Volume 1 is practical harmony, ornamental counterpoint and chorale. Volume 2 is modal and free style. See a complete review of volume 1 here.
YouTube is a great place to visit in order to see and hear many different organists improvise. Some even have their own channels. Listed below are artists and other YouTube channels that include improvisation videos as a significant portion of their content.