Wow, I'm surprised someone didn't already answer this. There is a file called itemslot.bin in the game. This binary file (which I haven't checked, but am sure it's encrypted) has item probability information. As of now, there aren't many people/REAL hackers that can read this file. I haven't bothered to look at it, but I'm sure it contains an array, or set of arrays containing item probability.
P.S. If you've ever wondered how Guru/God codes were created, it was via modifying the contents of Itemslot.bin. Basically, what these codes do is set your desired item to 100%, and all other items to 0%, therefore you MUST get it, and since you aren't technically FORCING the game to give you an item (old school item hacks), it is impossible to detect (assuming you use the code in the proper position, and don't go overboard, ie. blatant). If you want to find item probability: there are some sites with the table, but Zak's site: mkwii.org, has it... go check it out.
