You can find anything on teh internet these days. A quick web search at
http://search.yahoo.com/search?p=mia+key+largo+shuttle
came up with
http://www.floridakeysshuttle.com/
http://www.cwstours.com/flcities.htm
PLUS
Florida Keys & Key West Greyhound Shuttle, 3535 South Roosevelt Blvd., #104, Key West International Airport, Key West, offers seven-day service between MIA and Key West - both at the airport and at the corner of Grinnell and Caroline streets. The motorcoach line makes numerous stops throughout the Florida Keys & Key West; 1-800-231-2222 or (305) 296-9072.
Go Tours Airport Shuttle, MM 54 oceanside, Marathon, provides round-trip transportation between the Florida Keys and MIA and Fort Lauderdale-Hollywood International Airports; 1-800-689-3304, (305) 743-9876 or (305) 743-8446.
GREYHOUND KEYS SHUTTLE
The GREYHOUND KEYS SHUTTLE - (800) 231-2222 has three daily scheduled departures from the Miami & the Miami Airport to all areas in the Keys.
KEYS AIRPORT SHUTTLE
KEYS AIRPORT SHUTTLE 1-888-765-9997; Keys Airport Shuttle provides daily door to door shuttle service to and from Key West & the Florida Keys, Miami & Ft. Lauderdale airports.
Luxury Limousine of the Florida Keys, Ocean Reef Club, North Key Largo is a private limo charter service which picks up and drops off passengers anywhere in the Keys. Luxury Limousine operates on a reservation basis; 1-800-664-0124, in Islamorada (305) 852-4656, and at Key Largo's Ocean Reef Club, (305) 367-2329.
Super Shuttle, MIA, Miami. Super Shuttle serves travelers between Miami International and any location within the Keys, call 1-800-874-8885.
That should be enough to get you started.
Marc