Since you're in Brisbane, I recommend going to YHI Power (in Acacia Ridge) and getting an AGM deep cycle battery - see
this thread for details, specifically post #20. You can contact Bob (astronobob) for more if you need to. I picked up a 26Ah AGM deep cycle battery, brand new, for $50.
Put that in a plastic box for ~$10 or a specific battery box for ~$25 and then add whatever sockets or lug connections you need. I use a DC-DC convertor to drive the mount and wired that straight to the battery lugs via a fuse - I didn't bother with sockets. As Ken says, there are a few adapters at JayCar, but I got
this one off eBay for $16. Of course, with eBay, there's always the risk of getting a dud, but I haven't had any problems so far.
All up, that's less than half the cost of the Celestron Powertank, greater capacity and a better quality battery, too.