When I went to see Ayu in Summer 2008 for ASIA TOUR 2008, I used yahoo japans auctions. I typed in Ayu's name in Japanese and clicked search. I clicked around to get it to short the results by MOST EXPENSIVE. Then I used babel fish to translate information regarding the tickets. I went through
http://www.akibado.com/ - you have to transfer funds into an account on there and then tell them what you want to buy and how much you want to offer for it. It took a long time to get it though - I ordered my tickets in March or April and didn't get my tickets until 2 days into my Tokyo trip in JUNE (they sent them to my hotel). I got the tickets though so I was happy.
And honestly, if you are going - GO BIG. This is a once in a lifetime thing - so do it right. It will be worth it. I had to buy my tickets in a pair and I ended up selling one of the tickets outside the arena for about $100 American equivalent and I paid probably $1,800 American equivalent. I was 4th-6th row, center aisle next to the catwalk. It. was. HEAVEN.