If you go onto the main street of Akihabara (out of the station turn right if I remember, was last in there this September) on the left hand side of the street about half way down is a Mr Donut and there were a good 3/4 shops near there (some had pictures of it on sandwich boards) most of them I think were in those multi story buildings on the 3rd or 4th floor up. If you can see the shop that looks a bit like Super Potato on the main st with pac man and mappy on the right hand side then you are in the right vicinity and indeed if you go into the side streets on the left hand side of the street around there you will be near Super Potato itself...
In a lot of convenience stores you can even find Japanese guides on how to get roms and use flash carts for a few hundred yen to boot..
I saw on the news that the shops were advertising that they had it in stock. Many vendors would yell into the streets "R4 in stock. No questions will be answered regarding it. We all know what it does. WE HAVE 'EM IN STOCK! HURRY HURRY!" So if you understand japanese or even look at hobby shops' poster adds many of them actually have pictures of the R4 itself.
Got the card today. Asked many stores if they had it. Received about 100 ないです's. Finally saw a sign for it. Climbed up a narrow staircase and entered a room with this old dude in it. Walls were covered in 'panty-shot' pictures of high school girls. Was selling it as a combo w/ a 2gb kingston card. Very odd.