Hi guys. I usually get my music from
http://www.cdjapan.co.jp and yesasia.com, although I prefer CD Japan because the items are cheaper than Yesasia.
Other options would be
Amazon.co.jp and Rakuten.co.jp if you can read Japanese. Rakuten.com is also good.
But lately, I get a lot of out of print music CDs and concert DVDs from Ebay
