Sources?
What part?
The volcano duel comes from several interviews including one in August 1977 Rolling Stone.
Most everything else comes from too many sources to enumerate. I have the treatment and the different drafts of the original Star Wars script. I have the Brackett draft of TESB and the 3th, 4th, and published drafts. I have the published draft of ROTJ. Then there are all the interviews with Lucas in all the special features gathered in the various special collections. My list of sources is extensive and you would need to specify what you question if you want me to tell you where I found it. If I can remember or if I can Google it, I’d be glad to share.
Here is one for you. This lists the 1977 Rolling Stone interview and a 1978 interview where he hints that Vader might be Luke’s father - http://videoeta.com/news/1359/rumors-about-darth-vaders-beginning-have-root-in-the-past/
The volcano duel has been part of my Star Wars narrative for as long as I can remember. My mother relayed it to me. What her source was I can’t say and I doubt she remembers. She probably read it in one of the early interviews.
And yea, Frink, I have professional writer friends. All that means is that I know how stories are written and evolve. I’ve dabbled in it myself. I prefer to let other people do the work and study how they did it. I love hearing about the evolution of movie scripts, especially good book adaptions. I’m also good at investigating. Comes with my job. So I’m good at seeing patterns and possibilities. Seemingly small clues and add up to a definitive picture. I can’t say I am 100% right, but I am 100% convinced that Lucas had the basic story before 1977. Lucas was too good and too careful about keeping the secrets during the production of Empire and Jedi for the lack of these story points in the scripts to be considered evidence of anything.