jump on a JR train and head to Ochanomizu, it is also known as "Guitar heaven". Walk out of the station and turn left, within a 1 km walk there would be 20 shops. Some are on the main road, others down little streets.
You probably will need to go back a few times. I picked up a Dragonfly hi-sta when I was there in Feb
i was staying at shinjuku the last time i went but couldn't find any shops, how do u get to rock inn building? also how'd u get to the one at shibuya? the one there is ishibashi isit?