A "Cheaper" way is to use the Unearthed Arcana's Generic Warrior. You get all the bonus feats of the fighter, but they are not restricted to the fighter list. You can choose your class skills (and "good" save), and choose feats from a special list for generics (in addition to any feat out there). That special list includes three feats that have skill prereq's but give you sneak attack. So use 3 of your 11 feats for 8d6 sneak attack total. If you MUST have heavy armor, spend another feat (generics don't get heavy armor automatically). That still leaves you with 7 bonus feats, fighter BAB, 8d6 sneak attack. You do have to max out hide and move silently though, so that ties up your skill points. (Note: to continue the "thief" thing, you can buy evasion, improved evasion, trapfinding, etc., as bonus feats from this list, but the latter of course requires high scores in search/disable device).
I would think that, technically, you do not gain access to weapon spec., greater weapon focus, and greater weapon spec., since you do not have fighter levels. But maybe you could sweet-talk your dm.