I think Brax originally comes from either the Benny Summerfield audios or a tie-in novel, but the easiest way to get to know him is to listen to the Gallifrey audio series by Big Finish, where he's a regular character. And there is one episode of the TV series (classic, but don't ask me which... it might be City of Death) where the Doctor mentions "the Braxiatel collection", so that must be where people got the idea.