Seasonalrentals.com has monthly rentals.

Best time to get a 6 month rental is starting May 1st. If you rent for 6 months and a day you the 11% sales tax is not part of the rental amount. We have various options, please visit us at http://www.loffreno.com

Thank you Lloyd Nichols for mentioning http://www.loffreno.com, the best Fort Myers Beach Real Estate website!

Christian P. Loffreno, GRI, CRS

J. Capaldi: Fort Myers Beach does have 6 months leases for furnished rentals, however it is crucial that you contact the owners or rental agent way before the season starts at the end of December. Otherwise chances are these owners will switch to seasonal rentals with weekly and monthly rentals for example.
C21 triPower, or http://www.vrbo.com/vacation-rentals/region/usa/florida/central-gul… or

http://www.vrbo.com/vacation-rentals/region/usa/florida/cent…
or Lahaina Realty, http://www.loffreno.com/, craiglist.com and the local papers like The Fort Myers Newspress,the http://www.islandsandpaper.com/ which you can purchase, or the Fort Myers Beach Observer which has direct classified adds ( link below ) on the web page.( that is a good one )
http://www.fort-myers-beach-observer.com/
I live on the island and realy enjoy it. Let me know if you had an success finding what you want.
Lloyd

Yes there are many available. What exactly are you in need of?

Could you give me a zip code? You could drop me an email at elissee@gmail.com