00Machado said:
Any recommendations are where to find the pre-Mallory Arthurian stories?
Regarding what to take ideas from, I'm thinking the 1001 Tales, and Mythology. Pick any that interest you - Norse, Greek, Indian - all have themes and adventure hooks you can expand on.
Did someone ask for info about pre-Mallory Arthurian Literature? Yay! I can put my pre-Mallory Arthurian Literature class to good use!
For earlier quasi-historical sources of the ancient Celts, some of which briefly mention the tales that will be moulded into Arthur's legends, see Gildas's De Excidio and Nennius's Historia Brittanum. For the first glimpse of a more modern Arthur (plus King Lear, Cymbeline, and a bunch of other great stuff as free extras!) check out Geoffrey of Monmouth's History of the Kings of England. Umbran's reference was to Cullhwch and Olwen from the Mabinogion, which is also widely available.
To start seeing the modern Grail Epic take shape, check out Chretien de Troyes (his Knight of the Cart introduced Lancelot, and his Percival (along with crazy-weird continuations and retellings like Perlesvaux) was the beginning of the modern Grail story). For some fun poetic French stuff, Marie de France has a bunch of Lays, several of which deal with Arthur's court. The Pearl Poet's Gawain and the Green Knight is famous too.
For the big doozie--the source pilfered by Mallory to get Mort d'Arthur, that's called The Vulgate Cycle. Starting here, you begin to see all the familiar Arthur stuff. Notably, the author of the Vulgate Cycle seemed not to like Gawain, so you see a metamorphosis starting here of Gawain to a less successful or heroic character (in the Vulgate Cycle, he keeps accidentally killing the other Knights of the round Table due to mistaken identity and generally doing dumb things--oops!)