A source that isn’t accessible to outsiders, like your lecture notes, is viewed as personal communication. Cite the source in-text but not on your reference list, with the lecturer’s name as the author.
… as explained in lecture by A. A. Author (personal communication, Month Day, Year).
… according to B. Campbell (personal communication, January 23, 2015).
(A. A. Author, personal communication, Month Day, Year)
(B. Campbell, personal communication, January 23, 2015)
When possible, read the original text of a work before citing it. If you must cite a work that is quoted or paraphrased within another work, include only the work you read in the reference list. Name the original author in the text of your paper and cite the work you read.
… in Author’s work (as cited in Author, Year, p#) …
In Lu's study (as cited in Banks & Dudgeon, 2008, pp. 245-246) …