Unlike Western cultures which typically use a signature to validate an official document, Japan uses a stamp, called a hanko 判子to make a document official. There is very little you can do as a resident of Japan without one, so hanko is on the short list of things to buy when you arrive. Fortunately, hanko are available at many places, from your local stationery shop to your discount department store and yes, even vending machines. For a more unique hanko, however, you would probably want to use a hanko shop, where an artisan will design a hanko based on kanji (Chinese characters) or even your roman name.