I was reading in the March edition of Engineering in Miniature about making mini spark plugs. Due to the small size of model IC engines it is not always possible to buy a plug of the right size. So the author described how you can build a composite solution with ceramic components for hottest part of the engine and PTFE for further out which forms a good seal. The article also has lots of detail on how to machine the metal outer.
Commercial plugs are available but this was the only supplier I could find online in the UK.
Mini Spark Plugs from Just Engines Online Ltd
In related news via Slashdot, researchers are looking into using lasers to replace spark plugs although I’m not sure how this would scale to a smaller size.