So if I get this right you're running an AC source from your rig's battery to power up the camera's power supply which takes the AC power and makes the necessary 8 volts DC, and you're looking for a voltage regulator that makes the rig's available 12 volts DC into the 8 volts DC, correct?
If you have any experience with soldering you might wanna consider a kit. A resistor only drops the input voltage by a fixed value so voltage spikes and drops would potentially cause damage to your camera. A regulator provides your 8 VDC regardless of input voltage and your 200mA draw means that the regulator does not need to be beefy.
Or you can get one of this type of DC-DC converter -
http://www.powerstream.com/dc2.htm - model number PST-DC292 and set it for a common voltage rating of 7.5 VDC. Not the best solution but it should fit your needs if you keep the converter in a dry place, fuse properly and weatherproof your connections.