Yes, it will come with a USB charger and a USB to micro-USB cable.  If you have a car adapter that is USB, check the voltage to see if it matches the Nexus 7 specs.  If you are not familiar with AC/DC voltage and ratings, just buy a Nexus 7 car charger on Amazon, EBay, etc. You don't want to under/over charge your new tab and ruin the battery. 

They should be available pretty soon as after-market retailers will have any and every thing you'll need for this tablet.

Ugh....  its USB!  The voltages are going to be what the USB spec designates...   You're not concerned with voltages..   What you are concerned with is current (amps) .   The OEM charger for the Nexus 7 is 2 amps.   Your goal would be to get a USB outlet in your car that can deliver 2 amps.   Like the others have said, 1A should be a minimum.   The more you can get the better.   It will likely work on lower amperage models, but it will charge very slowly.. and your charger will likely get hot.