I think ssfas meant to say Disk Union in Ochanomizu. There are lots of musicians in that area and also a university. If you walk around you will see lots of stores selling musical instruments. I agree that area is excellent for record shopping. There is one Disk Union that carries all genres by one of the train stations. Both subway and elevated rail have separate stations there. There are also other Disk Unions in the area that specialize in individual genres like jazz, rock, etc. There is also an Audio Union (sister store of Disk Union) that sells equipment and is excellent if you are into tweaks and accessories. Japanese brands like Furutech, Oyaide, and Acoustic Revive available at much better prices than in the US.
The Disk Unions in Shibuya and Shinjuku are also excellent. Recofan and HMV in Shibuya are also pretty good.
One of the things that you will quickly realize about record shopping in Japan is that condition of used records is fantastic. Japanese take great care of their records!
If you want to shop for equipment, Yodobashi's main store has lots of equipment, accessories, tweaks, cables, and headphones. Dynamic Audio has incredible equipment, especially on their top floor which doesn't seem to always be open or may be by appointment only. Their other floors (I think there are 5), which are usually open, also have excellent displays of equipment.