PRIMARY SOURCES
http://comicsalliance.com/art-book-portal-2-dark-horse/
http://simoncini.cgsociety.org/art/cave-modo-photoshop-971677
http://www.thinkwithportals.com/coopsurvey/
Portal potatofoolsday ARG (PUT LINK HERE)
http://www.gameinformer.com/mag/portal2.aspx
http://www.lauradubuk.com/
http://steamcommunity.com/id/valve_artists/images/
http://www.cgsociety.org/index.php/CGSFeatures/CGSFeatureSpecial/portal_2 (if the full-res images do not load, append "cgsarchive/" onto the BASE url)
[Several images present are in SWFs that are no longer displayed normally without modification.]
https://web.archive.org/web/20130618222944/http://www.game-artist.net/forums/vbarticles.php?do=article&articleid=59
http://www.steamcardexchange.net/index.php?gamepage-appid-620
The Final Hours of Portal 2
http://cargocollective.com/lauradubukportfolio/Concept-Design
http://www.interlopers.net/articles/phil-co-interview
http://www.brennanletkeman.com/blog/2011/concept-art-portal-2/
http://tristanreidford.blogspot.com/2011/05/portal2-70s-style-movie-poster.html
Portal 2 Official Guide (Future Press)
SECONDARY SOURCES
http://animationtidbits.tumblr.com/post/46700357241/portal-2-concept-art
https://cazzablackabertay.wordpress.com/2015/09/22/case-study-portal-2-concept-art-resources-and-references/
http://www.gamer.ru/portal-2/kontsept-arty-portal-2
http://www.neogaf.com/forum/showthread.php?t=369866
http://combineoverwiki.net
https://www.hl-inside.ru/gallery/portal2-concept-arts/
http://agnmi.roflforum.net/t1366-cool-portal-2-concept-art

Thanks to the Combine Overwiki for locating these images:
Art\Cave Johnson\early_cave2_reference.jpg
Art\Cave Johnson\early_cave_reference.jpg