I know it exists in the country somewhere, because I ran a web search and found a ton of Japanese message boards discussing using it. (None of them say where they purchased it, though, they all start out, "I got my hands on some cream of tartar, and...") The ladies at the service counter of one of the supermarkets suggested I go to a store specializing in baking goods, but they didn't have a clue where one might be. ~_~ I guess I'll wait until the weekend and try Kyoto.
While I was out, I stopped at the post office to get a $3 money order required for a sendaway offer that accompanied one of the manga I bought. I ran into the problem once again that money orders are part of the post office's banking services rather than postal services, and their banking hours end at 4pm. Since I work until 5pm, and I only have a half-hour lunch break, I have absolutely no way to get there during their banking hours. (Actual banks, by the way, often close up shop at 3pm.) There are no banking hours on the weekend, either.
I already had the envelope filled out and everything, I just needed to stick the money order inside and mail it. I asked if I could leave the envelope and the $3 there, and they could stick the money order in the envelope and mail it the next day during their banking hours...but that is apparently against the rules. (Does it count as tampering with the mail even if it's at the sender's request?) After a lot of head-scratching and consulting various people, the plan we finally arrived at was that I would leave the $3 there, they would print out the money order tomorrow, and then they'd mail it back to me so that I could stick the money order in the envelope and mail it personally. ...I don't understand why it's okay to mail the money order to me yet not to the place I want it mailed...but oh well.