If you have not read Foucault’s main text on heterotopia, the easiest source on-line is to be found at Foucault-info – although there are better translations (see below). This text, entitled ‘Des Espace Autres'” was published by the French journal Architecture /Mouvement/ Continuité in 1984. It is based on a lecture given by Foucault in 1967.
Recommended reading list on heterotopia
An essay by Daniel Defert, Foucault’s long-term partner:
Defert, D. (1997) ‘Foucault, Space, and the Architects’ in Politics/Poetics: Documenta X – The Book, Ostfildern-Ruit: Cantz Verlag, 274-283.
Kevin Hetherington’s Badlands of Modernity – insightful and influential study:
Hetherington, K. (1997) The Badlands of Modernity: Heterotopia and Social Ordering, London: Routledge.
Casarino’s study of sea narratives uses heterotopia as a heuristic device to explore the tensions and crises at the heart of modernity (capitalism).
Casarino, C. (2002) Modernity at Sea: Melville, Marx, Conrad in Crisis, Minneapolis: University of Minnesota Press.
A series of essays (in both English and German):
Ritter, R. and Knaller-Vlay, B., (eds.) (1998) Other Spaces: The Affair of the Heterotopia, Dokumente zur Architektur 10, Graz, Austria: Haus der Architektur.
A series of transnational essays. This excellent collection of essays has a new translation of Foucault’s lecture and also a very stimulating piece by the editors, working towards a ‘general theory’ of heterotopia.
Dehaene, M. and De Cauter, L. (eds.) (2008) Heterotopia and the City, London and New York: Routledge.
Essay by James Faubion from the above is also spot on I think:
Faubion, J. (2008) ‘Heterotopia: an ecology’ in M. Dehaene and L. De Cauter (eds.), Heterotopia and the City, London and New York: Routledge, 31-40.
The English translation of Foucault’s lecture on heterotopia that I have referenced on this website:
Foucault, M. (1998) ‘Different Spaces’ (trans. M. Hurley) in J. D. Faubion (ed.), Aesthetics, Method, and Epistemology: Essential Works of Foucault Volume 2. London: Penguin, 175-185.
Most recent collection of essays on heterotopia:
Palladino, M and Miller, J. (eds.) (2015) The Globalization of Space. London: Pickering and Chatto.
Annotated bibliography of Foucault’s texts on heterotopia (listing both original French and English translations)
- Foucault, M. (1966) ‘Préface’ in Les mots et les choses, Paris: Éditions Gallimard 7-17
[The first mention of the term heterotopia, in relation to text by Borges, is found in the preface to Les mots et les choses]
- Foucault, M. (1968)  ‘Des espaces autres’, l’Architettura, 13: 822-23
[Some excerpts from a talk given to a group of architects, in which Foucault outlines the notion of heterotopia in relation to institutional and socio-cultural spaces, was published in the above Italian journal. The talk was a development of the second part of a radio broadcast he had given the previous year on the theme of utopias and heterotopias – see 9 and 11]
- Foucault, M. (1970)  ‘Preface’ in The Order of Things, Andover, Hants: Tavistock, xv-xxiv
[English translation of 1]
- Foucault, M. (1984)  ‘Des espaces autres. une conférence inédite de Michel Foucault’, Architecture, Mouvement, Continuité, 5: 46-49
[This is fuller version of the talk given to a group of architects, based on transcripts – see 2]
- Foucault, M. (1986)  ‘Of other spaces’, trans. J. Miskowiec, Diacritics, (16): 22-7
[first English translation of the talk]
- Foucault, M (1997 ) ‘Of other spaces’, trans. Lotus, in N. Leach (ed.) Rethinking Architecture: A Reader in Cultural Theory, London: Routledge, 330-336
[second English translation of talk]
- Foucault, M. (1998)  ‘Different Spaces’, trans. R. Hurley, in J. D. Faubion (ed.), Aesthetics, Method, and Epistemology: Essential Works of Foucault Volume 2, London: Penguin, 175-185
[third English translation of the talk]
- Foucault, M. (2001 ) ‘Des espaces autres’, Dits et écrits II, 1976–1988, Paris: Gallimard, 1571–81
[This is a tidier French version of talk in posthumous collection of Foucault’s writings]
- Foucault, M. (2004)  Utopies et heterotopias, CD: INA, Mémoire Vive
[recording of the radio broadcast – see 2]
- Foucault, M. (2008) ‘Of other spaces, 1967’ trans. M. Dehaene and L. De Cauter, (eds.), Heterotopia and the City, London and New York: Routledge, 13-30
[fourth English translation of the talk]
- Foucault, M. (2010)  ‘Les Corp Utopique, Les Hétérotopies’, with introduction by D. Defert. Clamercy: Éditions Lignes
[text of radio broadcast – an English translation is under consideration]
General Links: Foucault
Foucault Studies An ejournal providing a transnational ‘forum for discussion of Foucault which goes beyond received orthodoxies, simplifications and uncritical appropriations’. Free to register and download.
michel-foucault.com Extensive, lively and informative site created by Clare O’Farrell. It contains news of publications and conferences, bibliographies, a blog and much more.
Portail: Foucault Complete, authoritative French archive of Foucault’s work