This is one of my favorite stores in Miami, they carry Couture fabrics (cotton, wool, laces, silk), trims and buttons as well as some moderately priced fabrics. They have a bilingual staff, so you won't have trouble communicating, and the store is enormous. They established Isa in 1962, and the neighborhood has changed around them, it is not the most inviting, there is street parking, so bring your quarters. I've been there and have had no issues but this is the destination, it's not the place where you want to park your car and walk around looking for a place to eat, it's in and out. Although it doesn't look very large from the outside, it is two stories, they have an elevator to take you upstairs where the fabric is on display.
Isa Couture Fabrics
If you have a special project, or need to do color matching the staff is really helpful without being pushy.
This is the one I recommend if you have to prioritize your shopping in Miami.