Free Online Text: King Arthur, Tales of the Round Table
I found a free online text of the stories of King Arthur, complete with illustrations at Sacred Texts. This is an edition of the legends as edited by Andrew Lang, who was a prolific scholar and author.
I haven’t analyzed the text enough to tell what sources he uses, but it would appear to be based on Geoffrey of Monmouth‘s Latin work Historia Regum Britanniae (“History of the Kings of Britain”).