We have spend over a month in each of the past 2 years in a condo community on Longboat Key, a barrier island off Sarasota. Very pleasant without the third world "charm" of SE Florida.
Friends from the Lakes Region have rented from Naples & Venice in the South all the way up to St Pete areas North. Some have found places on Siesta Key, which is just below Longboat Key.
But time is critical. each year the areas seem to sell out for rentals quicker than before.
The agency we have used is the following:
RVA Resorts Vacation
(877) 415-4570
www.rvafl.com
Good luck!