I've had a look for you and it seems that the booster is not appearing on the US website at the moment...
I would assume that they are out of stock.
On the UK website they appear at the top of the list in the "accessories" section.
Perhaps Caz or Darth will have some info for you about a date for re-stocking this item,
or can move your question to the "ask TW" board for an answer on Monday...?