Specifically the Historia lists a series of battles it attributes to Arthur, and he is mentioned as associated with some of the Wonders. The best source for references to Arthur in early British literature is Coe and Young* (translations and facsimilies - though note, this is largely extracts, not complete works) but good websites include Thomas Green's Arthurian Resources, Mary Jones' Tŷ'r Beirdd or Celtic Literature Collection (the same site, but updated at different times), and the Camelot Project.