The most popular place to get a SIM card to use your cellphone in Miami is T-Mobile. They have one for $30 that will give you 2GB of data plus 1000 minutes for domestic calls.
Here are the stores in the South Beach area:
- 738 Lincoln Rd, Miami Beach, FL 33139
- 1507 Washington Ave, Miami Beach, FL 33139
- 1504 Alton Rd, Miami Beach, FL 33139