4th or 5th floor near the Sofmap Mac Collection? Sounds like Gamer Hollywood, which is also right by Messe Sanoh Chaos - the more reasonably priced import shop.
After you leave JR Akiba station cross the main street - Chuo-dori - at the crosswalk under the railroad bridge. Then bear right and head up the first street that runs parallel to the bridge. GG and MSC are both on the right side of that street.
On the other side of the main intersection (where the big new Sofmap is) there are some seedier shops that used to sell them, along the same street where DosPara is. March Rabbit, a.k.a. Sangatsu Usagi, used to sell them, but I think they've stopped since the crackdown.