You can get the H harness from Target, Kmart, Big W, Babies Galore. There are loads of places that see them now. I'm guessing the car seat fitting station would also stock them.

Anti-Submarine Clip acts to prevent the child from sliding under the lap seat belt on impact (Submarining). It also improves the overall performance of the seat by restraining the lower torso, reducing the likelihood of damage to vital organs.
Go to the Britax website, it's the proper one for safe and sound. Australian site. You find plenty of info on there.