The same chain that owns Takadanobaba’s excellent Thai restaurant Kao Tai (see the review in DOiT’s main site) has a well-stocked Thai grocery store, also in Takadanobaba. I buy there my Jasmine rice in 5 kg bags.
Where it is:
Walk from Takadanobaba station towards Waseda and after 300 m turn right into the small alley in front of the Wendy’s. The shop is on the right side of the street, about 50 m after the Billy Barew Beer Bar.