Looking for Guitar Freaks, Beatmania IIDX, Pop'n Music, Taiko no Tatsujin, et cetera? Look no further than the following:
www.ebay.com
www.ncsx.com
www.yesasia.com
shop.himeya.com
www.videogamedepot.com
www.ddrfreak.com (visit the forum's marketplace section)
www.redoctane.com (controllers, plus Pump it Up for PC)