See Teitaro Suzuki's classic text on sources about the first council. See Williams and Williams' review in Buddhism: Critical Concepts in Religious Studies on the second council. Since the third was called by Asoka (do you really need a reference? Even Clement of Alexandria commented on it). See Gutama's recent History of Buddhism in India for sources on the Fourth Council (the divided council). Since the Fifth and Sixth are so recent that even the European Press covered them, I know you do not need references on them. An easier way (if you have access to a good library with download privileges) is to look up each council in "Google Scholar" there are, literally, thousands of references and hundreds of non-Buddhist sources for the first two (the third and subsequent councils are so well documented, I cannot believe you can question them). Unlike the life or Jesus or the Besht, the life of the Buddha and Mahavir are pretty well documented -- in Hindu sources that really did not follow either of them. However, the Sanatana Dharma (or rather some of the more broad-minded of them) do consider both manifestations of this rather more ancient teaching.