This is the shopping arcade leading up to the huge
Nakano Broadway shopping mall - its just outside Nakano stations North exit (JR Soubu line). Unless you have two red hot steel rods stuck in your eyes, you cant miss it.
Nakano Broadway has 4 floors where the second and third are filled with Audio, darts, adult dvd, army wear, idol posters, figurines and a ton of other otaku gear. Its more likely that you'll find some of that rare stuff that you're looking for here rather than Akihabara.