NY, you really DON'T need special shoes for flats. You don't want too floppy, of course, but going too stiff may aggravate your ankle injury. A good walking shoe or light hiking boot is fine. I use my light New Balance hikers most of the time for backcountry work. Heck, in the summer, I've even used my Birks for quick rides, but my lug sole Columbia hiking sandals also work. In the winter, some folks up here even use Sorel pack boots.

There are special shoes for flat pedals if you want to spend the money, of course, but one of the big advantages of going with flats is that it gets you away from needing bicycle specific shoes. The only real advantage of flat specific shoes is the sole material that allows good grip with pins, but all kinds of regular shoes can do the same. Even my lugged hiking boots and lugged sandals have plenty of grab. Just a matter of trial and error with a given flat pedal to see what works.