Heaps of places sell it. We eat it all the time in my house. It is a fantastic grain and is the only one that is a complete protein.
Not sure where you are located but try:
Seijo Iishi (lots around town)
Kinokuniya
Natural House (Omote sando)
Kaldi Coffee Farm
Plus I would try a good depachika say in Shinjuku like Halc and Tokyu Hands.