wr_schwab has kindly posted a link to all the different places where you can purchase them. I've only bought railpasses once (until recently MrLapLap wasn't elegible) and Gendai have been good to me so I chose them.

Just pick whichever place is convenient to you, and try and pick a date when the exchange rate is favourable - around now is fine.

Bear in mind also that most of the Japanese companies will charge a 2% commission fee. You might want to pay in cash or see if someone like Trailfinders will waive it.