And here's the Epic feat:
Epic Feats :: d20srd.org
Hmm, a dragonwrought kobold rogue 15 could just pick that up at level 15. Possibly level 12 if he got Assassin's Stance and some prestige classes with +1d6 SA at level 1. Of course, that's a REALLY unimpressive feat and the requirements are nowhere near epic, it's pretty dumb it got that label to begin with.
Ok, here's some other options:
Backstab (dragon mag #340): Doesn't make the foe flatfooted, but gives you an AoO every time you flank someone and he doesn't attack you, so...you'll still get sneak attack on it anyway.
Mercurial Strike (dragon mag #310): You have to be unarmed and have Quickdraw feat, this feat then lets you draw a weapon when a foe provokes, and he's flatfooted to the attack. Just keep dropping your dagger after each AoO and draw a new one next time Or use a Quickrazor. Problems are..you won't be threatening much area while unarmed, and it seems like the kind of trick a group of foes wouldn't fall for twice.
Sneak Attack of Opportunity (dragon mag #340): Seems that the epic feat was reprinted. Requires SA +3d6 and Combat Reflexes, and only flatfoots foes to your first AoO each round.
Skill Tricks (cost 2 skill points, use once/encounter)
Sudden Draw (C.Scoundrel): Requires Quickdraw and 8 sleight of hand ranks. As an immediate action, you draw a concealed weapon when someone provokes an AoO from you, and he is flatfooted to the attack.