Other than random finds, there are a couple potentially good knives always in the game. There is one in a chest in the back of Kistandalius' shop in Ars Magicana, but you only get one brief change to get back there for it and the spell book. The other can be chosen at the end of the holy item quest (which occurs later in the game, requiring a visit to the castle, Verdistis and the Dark Forest).
There are many more good quality swords, which are much more powerful, though not as thief-like.
I found a few daggers worth a lot of gold, but never paid attention to the stats.
