Where are some book-offs in Tokyo?

I plan to travel to Japan soon and would like to get some cheap DVDs/CDs and magazines from the book-offs I hear about. I was wondering if people could list a couple cities that have them, and maybe mention what the biggest one is, or the biggest one you know of/have been to is?

Thank you :)

It is everywhere in Tokyo but top on the list is Akihabara Station branch. also Shinjuku Station East is as big.

Check this out:

http://www.bookoff.co.jp/en/index.html

In Tokyo, Akihabara store is big one I often visit at.
http://www.bookoff.co.jp/shop/shop20396.html
Oh, It says "the biggest store in Kanto the Metropolitan Area" on website.