It looks like special selection tickets (? fanclub?) are on sale now. Some of the retailers, i.e. Lawson, you can buy from a conbini, others you might be able to get online. Sorry I can't help more, AKB has a section for foreigners to buy when they're overseas, and the other times I've bought tickets I've been in Japan...
From my other post on another thread, (below, slightly edited) I'd like to add to try and contact fans/in Japan. Get on tumblrs, twitters, whatever - ask people to help. Say that you're willing to pay up front for the ticket and their time - it might be a lot easier than a middleman service, and cheaper than their fees too. You might make new friends at the same time and have someone to meet up with on the day!
Use this website -
http://www.celga.com/They can help you buy membership to the fanclub (it will use their address in Japan) and will also help you apply/buy for tickets.
Otherwise, offer to pay someone on twitter/facebook/etc. There's also no harm in going to unofficial overseas fanclubs or websites and asking for help - international fans may have contacts in Japan, so someone might be able to set you in the right direction.
Scalpers are an option, but your seats may be awful and sometimes they check for ID. If you get checked, you'll have wasted your time and money (and occasionally you may be put on a blacklist).
Finally, check on yahoo auctions Japan. If someone has bought too many tickets or can't go, they'll advertise them there. I think Celga can help you buy on yahoo auctions Japan. If not FromJapan is definitely able to help!
https://www.fromjapan.co.jp/2nd hand ticket stores (found in many major train stations) may also sell them. They basically buy them from people who can't go anymore/got too many tickets and sell them at an increased price. As long as you can say the group name, the date and place and ask how much, you'll be fine!
Good luck!
