If you’re serious about collecting Japanese music there’s only two things you need: an Amazon Japan account and your own Japanese address.
Amazon Japan is like any other online music store that sells Japanese music (i.e. CDJapan, YesAsia). You can create an account, order new CDs, and have them shipped to your door. Any of the other sites mentioned will do and are better for ordering in English. Amazon Japan does have an English site although parts of it are still in Japanese.
Why do I need a Japanese address?
Amazon Japan also sells discontinued (out-of-print) CDs through third parties. Sadly, they will not ship these items to the U.S. No problem. Tenso provides Japanese addresses and safely ships items to the U.S. Now you’re able to order rare CDs and save money on used items.
iTunes Japan is yet another source for new releases and out-of-print (OOP) music with the advantage of instant downloads. They may not have everything you need but I’ve seen their selection dramatically grow over the past couple years. iTunes Japan also requires a Japanese address. You’ll also need iTunes Japan cards available from JBOX or JAPAN CODES. (set up an iTunes Japan account)
Unless you understand Japanese you’ll have difficulty searching for artists and albums on the Web. Google Translate works with text and links. Google Translate is especially useful if you’re buying used CDs. Merchants list condition, imperfections, damage or missing items.