Almost exclusively from blog readers PM’ing me or emailing me with information, as I request in the blog guidelines. For Quit notices, they are usually the people quitting themselves. Very rarely, I notice someone banned; a little more often, I notice a “why was X banned” question before it gets deleted.
I have no direct or indirect communication line from Quora itself, and I have received no communication from Quora since launching the blog (including a response to my question about whether they had a problem with it).