I don't know of any Orchid Garden's in Miami but there is the Fairchild Tropical Botanic Garden They had a orchid show there just recently. I sure they have orchids there but not exclusively.
Dave is right, there are none. R.F.Orchids is such a nice place that it could almost pass for one. They also give a tour where they take you around the residential property as well.
I presume you may have been down here back in the spring and this info is late. If you are down here again in the future I will gladly show you around a few places.