Hanko is necessary for opening an account and it is good for gifts to your friends.
You can make stamps (HANKO) at Don Quijote (ドン・キホーテ)/ Takadanobaba Ekimae. A seal on which your name in hiragana, katakana or Kanji has been engraved is OK. Even if you do not understand Japanese at all, it will explain in English. (Cost: From 500 yen)