If you want an actual copy of Cubic Ninja (it is a 3DS game, but it is out of print), be prepared to pay a lot for it on ebay or something (maybe you're lucky and a game shop around has a used copy for cheap). With a Sky3DS, you can actually use a ROM of Cubic Ninja, and it will accomplish the same thing. That is all I can say on the subject, however, as linking to ROMs is strictly verboten here.
Use Google.
As far as ROM hacks go, here's a thread I found:
https://gbatemp.net/threads/play-rom-hacks-easier-hans-without-piracy-downloading.401162/
This might not make much sense until you actually set up your 3DS for homebrew, but just in case, here you go.
Finally, if you just want to do a ROM hack, and already have a legit cart, you can pick of a copy of Ocarina of Time 3D (there seems to have been a reprint recently) and a Powersaves 3DS to inject the save used to run the exploit. Using something called HANS, you can actually play ROM hacks on legit carts!