Shadow mode is THE fastest way to get money. You can rack up about 10k in 10mins or less. It wont be unlockable until after you purchase Mizuki at the store though. You need to buy Orochimaru to get Mizuki though... and he's about 60k. Best thing I can recommend to you is to go through mission mode on Genin, Chuunin and Jounin levels and earn yourself money that way.. if you beat all the missions in all difficulties it awards you 30k.