So I visited Japan for the Cherry blossoms earlier this year. While I was there, i bought so-net’s Prepaid LTE Data Sim card. Unlike b-mobile which needs to be pre-ordered and shipped to the airport, so-net’s prepaid data sim are sold either via a vending machine or a shop inside the Airport. Which is of course, extremely useful for people who decide to splurge on a data sim card in the last minute. Not to mention the data plans are the cheapest by far.
Official website: http://www.so-net.ne.jp/prepaid/en/index.html