Lactose in Tokyo?

[steamer]

Anybody know where I might find lactose (乳糖、milk sugar) in Tokyo?

I'm trying to figure out how to make kumiss or airag, a slightly alcoholic drink made from fermented mare's milk, no I'm not kidding. No mare's milk here for sure, but, I think if I cut cow's milk with some water and add some lactose it might work.