It's out there, but I haven't found it yet. Wonder if it's more of a Tokyo thing, I might have to head into the megalopolis and see, but $20 just to see if I can spot a bottle isn't really worth it yet. Incidentally, it's by Pepsi.
My friend was at school and some of her students came running up to her with a bottle of greenish liquid, begging her to try it. After trying to think of whether they'd just had science class and being generally wary, one of them took a sip. So my friend tried it and said it was "cucumber-ish."
When I find it, I'll have a ginormous photo posted.