No idea, but i'd be interested as well. I think APC used to have a miami store as a stockist but i haven't checked the site in forever. I'm going next weekend too, so i'd be interested if there was a cool shop to hit up. Probably won't find anything with stock better than NiCo, nicholas k, corpus, cheap monday, and the easy to stock stuff like that
There's a couple nice boutiques in South Beach but I haven't visited any of them, there's also a Co-Op close by in that same Miami Beach area. You might also wanna check out the Bal Harbour shops off Collins Avenue in Miami Beach, there's several upscale designer boutiques there.
If you have a car and want to drive 45 minutes, go to Sawgrass Mills, a mega outlet with huge inventory of quality clothing. Sawgrass Mills is in the town of Sawgrass and west of Ft. Lauderdale. I'm clueless how to get there via public transit.