Chris I picked up a Urban Battery pack (30 Ah) from ebay for I think AUS 350 delivered -
http://cgi.ebay.com.au/Urban-Basic-3...item4cec998efc
Which works sweet for the HEQ5. It may be a bit out of your price range but I invested with the idea of it being versatile for camping and when I eventually get more equipment (ie laptop etc) for my setup. It has sockets for 12V devices and other accessories included and is made by the same company that makes Thumper batteries. Hope this helps!
Darcy