Sarah,

The problem is that they didn't write much in 5th century Britain, and indeed the British language had mostly died out among the literate classes of society before anyone wrote very much of it. So your best sources will still be the early Welsh ones. Irish sources, as in Lady Gregory's Gods and Fighting Men or Cuchulainn of Muirthemne, might also supply ideas. Although the textual sources are late, many scholars believe the oral tradition they represent goes back to soon after Arthur's time.

best of luck!