I purchased a genuine 12v automobile adaptor from Neutron Express
for around US$42. Freight was expensive but they shipped to Australia very quickly. Good customer service. They were happy enough to deal with a small international purchase.
Power supply is branded INDA SD150A Specs are - DC input 12-24V 7A DC Output 12V 3A. It's big 4x2.5x1 inches.
Personally I wouldn't use anything but a correct power supply, these units may be rugged on the outside but the charging circuit appears pretty fragile. A friend ran his on an inverter for a couple of hours and the charging circuit blew. I would be very careful using 12v straight from cigarette lighter too, especially while starting and stopping the vehicle.
In the meantime i will see how longer trips go with a dedicated 12v converter