I think there's a tinder spark that will do this, but you can do it yourself w/o sparks. If you've got sapphire sparks, use the fractal cloud generator and make some moving fractal clouds. Then use these as a displacement map in action to create your heat haze. If you're feeling fancy, make 2 fractal cloud clips that move in different directions, and then use s_mathops set to difference on them before displacing. If you don't have sapphire sparks, you can always make a tileable cloud texture in photoshop and use that.
HTH,
Phred