I dare you to try Nakano Broadway instead of Akiba. It's like Akiba in a single building… it was actually Akiba before Akiba was Akiba (in terms of being a nerd mecca) and has the original Mandarake in it (though there are several of them there now - one for books, one for figures, etc). It's easier to get to if you're already in western Tokyo too - take the Chuo west from Shinjuku, 北口, through the covered shopping arcade, and there you are.

Just remember to take the stairs to the other floors. The ground floor is rather pedestrian, pun partially intended.