It isn't *necessarily* on the bottom level. Have played Wizardry for many years, actually the first time I encountered this dagger was on level *six*!The power of items that drop is directly connected to what kind of monsters you're fighting. Certain specific monsters have a small percent chance of dropping super-strong items, like this dagger. Basically you just need to wait it out and get lucky.
The other way to get a ninja is simply to have an Evil character and work him up to 17 of all stats, which allows for the Ninja class change. Takes a while, though...