Combustion? I think you mean Combust from the SpC. If not, then that spell, too. Also, Fireball and Lightning Bolt both explicitly mention they set combustible materials on fire and melt metals with low melting points. Burning Hands also mentions igniting combustible materials.
Don't bother with Orb of Force. Get yourself Gauntlets of Ghost Fighting from MIC (p. 216). All your spells and attacks will ignore incorporeal miss chances, so you can set ghosts on fire, too! Very good for a mere 4,000gp item.
A possible alternative to True Strike that I could suggest is Guided Shot. While the benefits aren't as great (ranged only, ignore cover/concealment/range increment penalties), it is only a swift action to cast, so you could use both it and an orb or ray spell in the same round.
While not a direct method, there are some spells that themselves burn when subject to fire, such as Web and Incendiary Slime (CM p. 108). I dunno if those are something you'd be interested in, as well.