Depends on do you mean 240volt to 12v
or do you mean 12v as in car battery ?
240v-12v examples =
http://www.ripmax.com/item.asp?itemi...0&Category=070
http://www.ripmax.com/item.asp?itemi...0&Category=070
or 12v's
either go visit your local halfords or car parts stockists and try to get a good deal on leisure batteries or failing that a battery stockists ( ats ,kwikfit) and get a decent 12v tractor/truck battery