There are several places the offer realistic flame paint jobs, a trend started by Mike LaValley (spelling?). He's up in Canada or Alaska, but he hand air-brushes the whole flame with just a freestyle curve board. I'm sure you can find several pictures of work he's done, and maybe find a picture of just the fire itself.
Googling for "fire" in google images might work as well.
Here's a link to one of the hotrods painted by Mike LaValley.
http://www.adata.com/~aod/flame_job/42317.jpg
http://www.adata.com/~aod/flame_job/42318.jpg
http://www.adata.com/~aod/flame_job/42319.jpg
and I just found Mike's site (It's Mike Lavallee).
www.killerpaint.com