You need a voltage regulator. Google for '7812' and 'voltage regulator'. A manufacturers datasheet will give you all the info you need.
If you need the 12V ONLY for the LEDs (no other electronics that needs exactly 12V), you could do without a voltage regulator. LEDs will work on any voltage (DC). In series with the LED is usually a resistor (I think, in your case, about 470-1000 ohm). All you need to do is increase the size of this resistor by 18/12.
But, if there's ANY electronics there that needs 12V, go for the voltage regulator solution. It's not that hard to build, as far as electronics goes. I suppose you know how to hold a soldering iron, if not, find someone who can (HAM, local TV-repairman, etc.)
I don't know how much current these igniters use, but the 7812 can supply max. 1A (some special versions can go up to 1.5 and 3A)
My advice remains: find someone local who can help you out. If you ply him correctly, he might even build the complete thing for you.
Given more details, a better answer could be given.
Peter.