Okay, I've got some info for you on those 2 music stores in Shinjuku.
The first one's called Ishibashi Music. Here's their website:
http://www.ishibashi.co.jp/
To get there, exit out of Shinjuku Station's East entrance (higashi guchi). In front of you will be a main street. Take that street to the right and walk for about 5 blocks. The store will be on your left. Across the street will be Marui City (OI City) department store and the Wald9 Cinema. Here's a pic of the store:
The second store's called Rock Inn. Here's their website:
http://www.rockinn.co.jp/index.html
To get there, exit out of Shinjuku Station's East entrance (higashi guchi). Immediately to your right will be a Yodobashi electronics store. Make a right at the Yodobashi store. Take the side walk down 2 streets and make a left at the United Colors of Benneton store. About a block and a half down there will be a McDonald's on your left and a Wendy's right across the street from it on your right. Keep walking past those restaurants and make a right at the next corner. One street in front of you should be the Rock Inn store. It's across the street from the big IDC Otsuka furniture store. Here's a pic of the store:
