Hey mr. newbie.
Most large HMV and Tower Records stores in Japan have game music included in their soundtracks section. Large electronics stores like Yodobashi Camera and Sofmap may also have a small selection of the latest releases.
If you're looking for out of print stuff, search for any Book-Off outlets near where you're staying: this chain of second-hand stores commonly has a game music section. And if you're really serious, get yourself to Tokyo and dedicate at least a day to Akihabara and Nakano where you'll find many, many stores in which to haemorrhage money on game music old and new
