If you have a question about the source of some information, and the information is not provided on the website, I would contact the Wolfram|Alpha team about it.
http://www.wolframalpha.com/contact.html
When using Wolfram|Alpha there is a form at the bottom of the results page for contacting them.
More complicated operations are usually the results of some computation done on other sources, so it might be more complicated to tease out what the data sources are.
