There was a thread on jdmu that mentioned some good ones but I cant find it. I would try the google maps "find business" feature for miami. Look up words like "tuner", "tuning" "motorsports" "racing" ect. and that should point out some shops. Then you can make some calls and have those shops recommend others.
Also, unless your looking for something specific, theres not much to buy IMO (as opposed to a trip to japan for example). In the US we spend a lot of times trying to find all the parts already on your car!