Buying JR Hokkaido Free Pass in Tokyo?

Hello,
I'm planning a trip to Hokkaido this July using JR Hokkaido Free Pass (7days).
I will take a night bus from Shinjuku to Aomori.
I want to use the pass to travel from Aomori( Nakaoguni) to Hakodate.

Since I'm not sure whether the pass is available at Aomori Station, I want to buy it in Tokyo, before taking the night bus.

My problem is I can't find information about where can I exactly buy the pass in Tokyo.

I'd be appreciated if anyone can help me, or give me a link of :
1. ) where are JR Hokkaido Office Branches located in Tokyo.
2. ) Which JR stations in Tokyo can I buy this pass.

Thank you very much for your help.
Lily

It seems that there is a "JR Hokkaido Plaza" in Tokyo Station, where you might be able to get the ticket. It is located near the Underground Marunouchi Central ticket gates:
http://www.jreast.co.jp/e/stations/img/map_e/e1039.pdf

Thank you!