LED drivers/transformers are widely available but they however do the opposite job to what you want, that is 12V->230V. Ones that step down to 12V are also available but you'd have to look.
Best bet as i mentioned before are the MR series of bulbs (most common of which are MR16) or 60 5050 led/meter (300/5 meters) strip lighting (which has roughly an input of ~14W/meter). Both require a 12V power source. Fitting the strip around the perimeter of your ceiling will give you reasonable light output and can be wired directly to your battery, omitting the 12V adapter (which costs extra anyway)