The tickets for April probably are not available yet. The general sale for the CDL doesn't begin until 27 November:
http://www.avexnet.or.jp/ayu/news/041022.html
Keep an eye on her official site for updates.
I don't know anything else about getting concert tickets in Japan, except that Ayu tix are always available at yahoo auctions japan just before the concert date (possible last resort).
I hope you get to see the show, good luck!