when i went to cdl 2010-2011 i bought the ticket in a re-seller shop in shibuya that is very easy to find and where you can buy many kinds of tickets, but be prepared for VERY expensive prices if you want to be near the stage! this is their website ->
http://www.yokohamaticket.co.jp/tick...cket-music.cgi
if u go to the shop they'll have more tickets (they dont update this website too much)
the other option is to buy them in yahoo auctions japan but i have no idea how to do that :S xD
good luck!!