Question... are these for a stabbing, or for a more in depth fight scene... slashes and thrusts as well as stabbing.
Depending on what you're doing, you might get away with showing the knife, then having the actual stab off screen. If your going for a life knife fight, or what have you, you might look into martial arts weapons places on line for their practice knives... though I can't guarantee how those would photo either.
If you know someone crafty, you might get a real knife, then get them to make you a wood or dense foam copy. Then the real knife is available for close ups or can be used to stab an object like a couch, but the fake knife is what actually is used when there's even a reasonable risk of injury.
I know that for most fight scenes involving swords/knives they generally have a bunch of different versions of the weapon. A sharp "Hero" blade, a dulled metal blade for if the two meet (Choreograph VERY carefully), with wooden and foam versions for actual "Hits".
Keep in mind, though, that Hugh Jackman has, apparently, stabbed himself in the legs multiple times with Wolverines "Sharp" claws...