Yes, it's time for yet another Mint location. There are very few card stores in Japan that aren't part of the Mint chain, though I discovered a really great one yesterday - I'll be telling you about that soon enough!
Mint/Yellow Submarine Akihabara is pretty easy to find. Go out the Electric Town exit, to the left, and make a right towards the major street (Chuo Dori - LABI should be on your left and you are heading towards LAOX). You want the building on the left corner, which should have a bunch of display cases with anime stuff inside it. Look for Radio Hall Of The World (or a similar sign). Note that there has been a lot reconstruction in the area lately, and Google's street view is out of date. You want the building that looks abandoned with the red overhangs in street view (now it has yellow signage). The cards are on the fifth floor of the Radio Kaikan building.