Most of the time they freak out and start screaming. That is when I brandish my weapon at them and say "we shall meet again under the cover of darkness"
Is this the punchRobert stuff? Like Urban Ninja? That stuff is hilarious. He is why I got into doing that to people.

I think I saw some on ebay, which is where I got most of my kunai and shuriken.
I have become pretty proficient with them, kunai are harder to throw but because of the added weight they go in farther and so do more damage.
Or you can usually find a knife/swords shop in a mall. Every mall I have ever been in has had at least one tucked back into a corner by a comics book store.