+5V is available on the PS/2 keyboard connector if the laptop has one.
USB connectors are the only other peripheral connector with +5V available at any useable current.
Otherwise you'll have to dig around in the laptop to find a +5V source either on the motherboard or on the leads that go to the LCD. The latter would probably be better since you mentioned the screen is broken.
I wouldn't use a battery or the car if there's already a regulated +5V available.