Car Battery power supply questions

So I will be using two maybe three arduino boards in a car for various temperature sensing, rpm, speed, fluid levels...etc. etc. I am curious what is the cheapest and most efficient way to supply a constant voltage of 9 or 12v. I'm assuming hooking them straight to the battery is a bad idea due to the large voltage spikes/drops and high amperage. With the alternator car batteries make about 14-15v which would be above the recommended 7-12 volts anyway.

I could do DC12V>AC120V>DC9V but that seems like a big hassle, lots of space taken up, and probably not too efficient for such a small load. Is there a good 12(15volt) DC to 9/12 volt power supply? Or do I need to build one.

The recommended 12V max is only a heat dissipation thing. If you are not pulling much current through the 5V regulator it should be fine.

There's always the option of the ~$2 car charger for USB devices that fits in the lighter socket.

There are plenty of devices which convert 12V DC to 6V DC, and use a switching circuit so they are quite efficient, and about $4 on ebay.