Personally, I'd say resellers shops. Go a couple of days before the concert and there should be enough tickets available to choose which one you want (or, how much you can afford!).
The last couple of years I've got a B-Block ticket for around ¥15,000. This is on the arena floor, and close to the centre circle of the catwalk. Plenty close enough for me.
Then there are tickets for the stands and further back on the floor which are a lot cheaper, and closer to the official sale price.
The website I use is:
http://www.yokohamaticket.co.jp/ticket/ticket-music.cgi
Check nearer the time, and they'll start to list the stock they currently have. Some tickets are sold in pairs, but they'll probably split them into singles if you want one.
Hope that helps!