Oh and building your own is easy
Coda batteries in sydney often have secondhand batteries from UPS applications. Most of these batteries have been kept on a maintenance charge have never been discharged at all and are replaced (dependant on application) every 6months to 2 years.
I picked up a 100Ah deep cycle for about $90
Buy yourself a good battery box (i build mine from MDF as i couldnt find one big enough for the battery) fit the battery and run a charge line (8awg cable) from your main battery (with or without a dual battery isolator - i use a redarc isolator available from sidewinder.com.au) to a 50amp anderson plug and fit the other anderson plug to your aux battery box. Remember to fuse it with a fuse at both ends rated at no more than 50amps (as thats what your anderson plugs are rated at).
I personally use a circuit breaker at either end instead of a fuse (they do essentially the same thing but the circuit breaker is easier to cut the feed from the main battery when im stopped over night or removing the auxilary battery to use away from the vehicle) get a MANUALLY resetable circuit breaker automatic or self resetting are no good in this application (IMO shouldnt be used at all ever) you can get these from jaycar DSE or any car audio store. then fit a couple of cigarette lighter sockets or hella sockets (I use 2 of each) this way i can run my fridge and a 12V light my mobile charger and i have a spare socket for on totalfire ban days when i use an electric stove (these are awesome on TFB days - cook pies and hot dogs and reheat meals brilliantly!)
If you want I'll draw some pictures or have a look at http://www.sidewinder.com.au/product2.html
and get an idea or buy a ready to go kit.
Derrick from sidewinder is a great bloke to deal with very helpful and ships his products extreemly quickly! I purchased my redarc SBI12D and 4x 150amp anderson plugs from him and recived them within 3 days!
He also lists his items on ebay (how i came across him actually) and you can get many of his products cheaper through ebay.