Ways to get more sneak attack, albeit by watering down your build a bit: dipping Assassin 1, Spellthief 1, Master of Masks 1 (for Assassin Persona Mask)...
Sudden Strike might be OK as well, since your giantslaying powers will make foes flat-footed, meaning you don't rely on flanks too much. So dip that Ninja, Binder (Malphas), etc.
But I'll suggest something totally different now. Forget about sneak attack. Cram in just one more feat (which you can do putting some Fighter, Feat Rogue etc. in your build), take Leadership, and get yourself your own personal dragonfire inspiration Bard. Now you can roll a truckload of d6s on every hit, regardless of flanking/flatfootedness!