Where to buy cosplay costumes in Tokyo

I decided last minute to go on a halloween event in Shibuya, so online would porbably take too long. Is there a good shop in Toyko where I can get good cosplay costumes?
I think Akihabara is a good spot for such things, but I wouldn't know where to start to search there.

Can you also buy somewhere the "tomodachi mask" from the 20th century boys movies, since that would be a really easy costume?

There's a place that sells them just near Akihabara station.

If you take the JR Yamaonote line from the direction of Tokyo station and look west out of the window just as you get to Akihabara Station, you can see the shop. It has around 4-5 floors of displayed costumes visible from the station platform.