Another good source is CraigsList.
In addition, local real estate investor clubs often can help. Here's a link for clubs in Florida: http://www.creonline.com/real-estate-clubs/fl.html
And consider direct mail--primarily postcards--to areas of potential renters.
Terry's suggestion regarding yard signs is good; I've heard that they can be very effective. I've also seen "bandit signs" used to attract renters and tenant-buyers.
Hope that helps.