Some are reported to the Multiple Listing Service (MLS for short). Some are rented out by the owners privately or some use a private rental agency. So not all properties are reported to the MLS.
There is not a public site that I know of to look up rentals.
As realtors we can find it on our multiple listing service, so if you needed a specific home or area researched you could ask your realtor to find that infomraton for you.
All the best,
South FLorida Brokers