Joker boot knife from Dark Knight?

If you've seen the movie "The Dark Knight" you will know at one point, when they are at Harvey Dent's birthday party, the Joker and his croonies come in. At one point to attack Batman the Joker makes a small blade pop out from the tip of his shoe. I was wondering if anyone knows if any are for sale or how to make one? I have already found a video of a person that made one and tells you how it works, but he doesn't give instruction on how to make them, any ideas anyone?