Gordon Cooper:
http://www.openminds.tv/gordon-cooper-ufo-080210/
Buzz Aldrin:
http://www.openminds.tv/nasa-astronauts-disclose-ufo-encounters/
Edgar Mitchell:
http://www.ufoevidence.org/documents/doc507.htm
Story Musgrave:
http://www.opednews.com/articles/Confirmed-Top-NASA-astron-by-Terrence-Aym-101028-932.html
Neil Armstrong:
http://drgreersblog.disclosureproject.org/?p=202
Note that this was a quick and dirty link search. Curiosity about any of these events will easily lead to other sites and the videos of the same people speaking out on Youtube, various documentaries, books and multiple printed interviews.
The topic is so massive and explosive in its scope that it boggles the mind. The only thing I'm sure of is that these men (and many other of their colleagues) didn't suddenly lose their marbles and get together to cook up a bunch of pranks for personal amusement. In my opinion there could not be a more qualified class of observer to comment on the topic and comment they have.
Edgar Mitchell is probably the most candid, articulate and widely quoted guy out of the bunch and there are pages and pages of his views to be found on Youtube. He says whats on his mind.
The two best books I've read on the topic are The Day After Roswell by Lt. Col. Philip J. Corso, which blew my mind, and UFOs: Generals, Pilots and Government Officials Speak Out by Leslie Kean, which is an impressive recent piece of serious journalism on the topic.