That makes your search a whole lot easier.
First you have to go to this store:
It'll have pretty much anything your looking for there:
To find it, hmm, look for the big red Club Sega when you come out of the station. Its in the area behind it. Also notice the 2 in the sign, there are 2 other store locations but sell other computer stuff. If you find one of them, the other ones are relatively close (same street I think).
Though if you are looking for deals then there are other stores (though the one above probably has the best prices on Realforce boards).
The best price (since I last checked a couple weeks ago) for a black MX Red board is at SofMap (right next door).
Best price for a white MX Red board is at T-Zone (DIY Shop), which is in approximately the same area (that's where I got my ¥3980 deal).
There are ANSI keyboards available for pretty much all of the worth while keyboards (uTron being an exception, but hey, its a uTron). So make sure what version you are picking up (easy way to tell is by the number of keys the board has).
You'll probably want to check out Yodabashi-Camera just cause its enormous, but it also has a decent keyboard selection (no deals really, though). Also has a good mouse section.
Also, lots of the shops that cater to PC Gaming also have mechanical keyboards, but their prices and selection aren't as good (though you can get lucky).