given your size you are going to pay a hefty price for any bra.
My advice is do not go to Bra n Things. They suck. And they will sell you an incorreect size just to sell a bra.

Myer and David Jones are definatley worth a shot. And they will have a large selection.
The other thing is to try looking for a plus size bra store, that sell only DD and bigger. They exist! lol

Good luck hun
If all else fails, have a look for some Hot Milk stuff online, they are awesome And once you have been fitted correctly, then you can order with more confidence