You can start with modern novels and and films, which often contain much invention by the modern authors. The film “First Knight'' in particular is one where much of it is simple invention found in no previous Arthurian tale. The film “King Arthur” has even more modern invention in it.
What do you expect from Hollywood?
Or you can start by reading the genuine medieval tales, These are also mostly invention, in that the tales they tell never really happened, but they are at least the authentic medieval tales, rather than modern inventions.
For medieval tales in print in modern English see:
http://www.amazon.com/Medieval-Arthurian-Modern-English-Translations-%231/lm/R3KPSDORBZLMJ6/ref=cm_lm_byauthor_title_full
http://www.amazon.com/Medieval-Arthurian-Modern-English-Translations-%232/lm/R2S7V81SU470N6/ref=cm_lm_byauthor_title_full
http://www.amazon.com/Medieval-Lancelot-Modern-English-Translations/lm/R2SR3FRVTQ2CIU/ref=cm_lm_byauthor_title_full
http://www.amazon.com/Medieval-Tristan-Modern-English-Translations/lm/R3JLROH1RM9MI0/ref=cm_lm_byauthor_title_full
Malory’s “Le Morte d’Arthur” is generally the source for most modern English efforts.
In the 19th century Tennyson's “Idylls of the King” had an impact equal to that of Tolkien’s fantasy in the 20th century.
Twentieth century reworkings recommended by many are:
T.H. White’s “The Sword in the Stone” and “The Once and Future King”;
Mary Stewart's “The Crystal Cave”, “The Hollow Hills”, “The Last Enchantment”, “The Prince and the Pilgrim”, ”The Wicked Day”;
Marion Zimmer Bradley's “The Mists of Avalon”;
Thomas Berger's “Arthur Rex: A Legendary Novel”,
Bernard Cornwall's “The Winter King”, “Enemy of God'', and “Excalibur”.
Films about Arthur have not been very successful, save for “Monty Python and the Holy Grail”, which is a magnificent parody. But John Boorman's “Excalibur” is generally recommended as a good reworking of the medieval tales (mixed with his own invention).
Note that some of the above material is available on the web. See http://www.lib.rochester.edu/camelot/mainmenu.htm .
Note that there is no single accepted Arthurian continuity. Even medieval works purporting to cover the same events are often very different from one another.