For those who want to see Ayu at the Yokohama Arena:
This seating map, which I made, is probably not 100% accurate (especially when it comes to the blocks around the lower half of the circle) but it should give people an idea which seats to choose, if the stage design is the same like in 2009 and previous years.
I also scanned a part of my ticket and marked my seat in the seating map.
Generally, I would recommend buying A2, A3, B2 or B3 tickets at the Yokohama Arena because you will be quite close to Ayu.
Searching for tickets on YJ!A:
Last year I used those keywords to look for floor tickets at the Yokohama Arena:
Ayu's name in Kanji + concert date (M/D) + floor seat (in Yokohama they are called Center/センター but in other arenas like Yoyogi the floor seats are called Arena/アリーナ)