[Sangetsu]

Do as MMM said and just buy a ticket at the station before you leave. I have never had a problem getting a seat on the Shinkansen, even when buying my tickets at the last moment. If you are still worried, you can buy your tickets a day or two beforehand. At worst, you may not be able to get a reserved seat, and non-reserved seats are first-come first-served, so get to the station early.