www.hmv.co.jp is a very handy website to check out before visiting an HMV in Japan. You can check a specific item and see which Japanese locations have that item in stock, low stock or not in stock; the inventory gets updated every hour. You can also reserve a copy at a particular store if you create an HMV account and have a Japanese home address. The three items I desperately wanted were all listed as 'low stock' at the Lumine Est location, and about 24 hours after I purchased these, they were listed as out of stock. If you see an item listed as 'low stock,' I suggest reserving a copy if you can and picking it up quickly!
Overall, I absolutely love Japan's HMV and the inventory system and would love to shop here again someday!