So I was talking to AstroStar7 and she directed me to this guide by JDaster: http://www.gamefaqs.com/wii/942008-mario-kart-wii/faqs/64637 It has stats pulled directly from MKWii's code for Characters, Karts and Items. I'll be submiting this to the guide section and I'm waiting to hear back about how often items can be pulled.

Does anyone have an accurate source for how items are distributed, what places you can get them in, how often they can be pulled and how rare they are?

I can't answer the second question, but an ISO is either playing the game on an emulator or loading it onto your wii from a USB.