While you can buy shelves in Japan, I didnt realize just how uncommon they were. One of the reasons is that most people in Japan rent an apartment - and if you are renting then you are not allowed to make holes in the wall - or the wanker of a landlord is going to charge you something like
20000 yen from the 2 months rent deposit of yours that he has (I was charged
16000 yen for a small scrap of peeling wall paper in on of my
previous apartments).
Now even if you do decide to make holes in the wall (and try to patch it up with toothpaste when you move out), the walls in some of the apartments to rent are so thin that it wont take the weight of the shelf. Apartments in Japan are generally built to below standard - from the landlords point of view - they just want to spend as little as possible on apartments but charge you an arm, leg and maybe a kidney too.