We have used the following agency for the past 3 years for renting a condo on Longboat Key:
RVA Resorts Vacation
(877) 415-4570
www.rvafl.com
You will need to act quickly since the Florida vacation rentals, particularly on the SW coast, tend to sell out quickly. Lot of people from this area go to Bonita Springs, but we prefer Longboat Key, off Sarasota. Others we know from this area rent in Venice, Siesta Key, etc.
Good luck.