Electronic modules that take car 12V and make 5V are sold at every convenience store, gas station and grocery store in the developed world. Right next to the phone charging cables. They're not even buried back next to the transmission fluid, they're usually right next to the cashier where you can't miss them. They are possibly the most popular electronic product on earth.
All you need to do is provision the things that go on either side of the module: a car "cigarette lighter" type 12V receptacle, and put a "USB plug" on your Raspberry board. The USB plugs are sold right next to the modules at the same endcap display rack. For the cigarette lighter receptacle, you'll have to scour the earth for any auto parts store, Radio Shack or Walmart/Kmart might also stock it.
The reason to bother with a cigarette lighter receptacle is so if the unit fails, you can swap it in 10 seconds in the parking lot of the convenience store where you buy its replacement.
Your ignition has several positions: Off/Lock, Accessory, Run and Start. Accessory is an intermediate position on your ignition switch made for sitting in your car listening to the radio, without engine features, fuel pumps and cooling fans running down your battery. Most people don't even realize the "Accessory" position is even there.
Your car's fuse block may have spare terminals specifically to tap for things like this. Some of the circuits will be always-hot, some "Hot in Run" (but not start), some "Hot in Run/Start", some "Hot in Accessory/Run". You'll have to decide which circuit is practical for you to tap, and which suits your purpose. "Hot in Run" will lose power for a few seconds while you crank the engine.