The simplest way to go would be 12v to 240v DC to AC inverter (can be bought quite cheap) and factory made AC adapter with dummy battery (quite expensive but reasonably safe). This set is not terribly energy efficient, but with the large battery that barely matters - and it can be used with car battery. 9v supply is risky - it may shorten camera electronics lifespan, especially when used in summer warmth. Also, see this:
http://www.camerahacker.com/myink/Vi...ital%20Cameras