Like I say, I bought mine from my second-hand store for £25. I was very lucky to find it, because it's not something you see on the shelf every month, let alone every day.
All I could suggest to you is to visit your local second-hand game stores and check back every week or so.
Also, as mentioned by Clydefrosch, other exploits such as Browserhax, themehax, and ironhax have been updated for the latest firmware, so you might not even need to get Cubic Ninja again.