The typical approach is to use an off-the-shelf 12V inverter connected to the 12V lead acid battery. With the car turned on, the DC-DC converter charges the 12V system from the high voltage battery. For more common cars, you can buy bolt-in kits for exactly this purpose:
http://www.evextend.com/Emergency-Power-Kit.php
Perhaps you could contact that vendor and see if they're interested in making a kit for the Spark EV. While it's not an extremely popular car, you can often convince small businesses like this to make a small batch if there's a guaranteed minimum ("group buy") quantity that are paid for in advance. Several people have discussed it on this forum, so you're not alone.
Bryce
