I bought mine from a local computer shop for my HP which is 19v it is made by a firm called Vanson and not only will it manage the correct voltge by it will also cope with the fact that my laptop needs 120w, it just plugs into the cigar lighter socket.
Whatever you use to convert the 12V car battery power to laptop power, if you reckon on about 90% efficiency, divide the power used by the laptop by 12, and add the bit extra for the 10% wasted power, you will get the car battery current drain. This always comes out at a surprisingly high figure. Fortunately the laptop doesn't run at the peak power all the time, I would guess at an average of 50W - but that is just a guess - which equates to about 6Amps. If your car battery is 50 A-H, and it is fully charged it will have about 40 A-H (car alternators never charge the battery to the full rated level to avoid overcharging), and you don't let the battery discharge below 50% or you will have to push the car to start it, then you have about 15 A-H maximum to use for the laptop. Keep the usage without the engine running to less than 1 hour and you should be safe as long as the battery is fairly new.