Any luck with your search of a mixte in these sizes? This forum is definitely a good start. They're indeed quite rare, especially 56" frames. I did a national search and found only three 56" frames. There is just one worth mentioning. It's a Concord in Pittsburg. Even though Concord is not a renowned brand, the bike is made in Japan. I like the chrome tips, it has some beautiful logs and the color seems quite decent. Definitely worth the $120 but you could for sure negotiate. You should be able to find someone here living in Pittsburg to sponsor you with the purchase.

http://pittsburgh.craigslist.org/bik/1748372317.html