Seeing as how you're in Japan, you probably have access to a bunch of mid-quality-or-better 70s-80s mixte bikes. Most Japanese builders had a mid-range mixte even for the US market, and my understanding has always been that they were much more popular in Japan than they ever were over here, so your odds of finding a good one are probably pretty good.. They're all gonna be steel, but they're also gonna be lighter and nicer and better than any aluminum step-through hybrid you're likely to run across. You'll most likely end up buying a complete bike, I doubt many were sold as framesets.

Other than that, there are plenty of custom framebuilders in Japan that could do you up right. (Unlikely to be cheap, though...)

--Shannon

Living in Japan and you are asking about a step-thru frame. I live in Cambodia and there are more Japanese Step-thru bikes from recycle shops here than I can buy.