Our LCD TV is a Sharp 15", it came with a mains adapter as it is made for 240v, all I did was looked at the rear of the TVs in the shops and most have a small jack plug which has the voltage moulded in around it, and a transformer to give a DC voltage.
Some are 15v, some are 9v, different makers use different voltages, our Sharp, some Panasonics and maybe others are 12v.
A 12v battery gives a voltage near 14v when in a usable charged state and around 14.8v when on charge, dont be put off by this as the transformers supplied give over 12v as most 12v appliances have a tolerance from around 10v to 15v. Neither mine or the Panasonic 15" LCD used by my parents in their boat with digital charge regulators giving a constant 14.8v has ever shown any problems. If you buy a dedicated 12v TV you will pay well over the odds for a cheap make, save your money and buy a decent normall one, mine was a