Speaking as a home owner, we have a management company where the boss was born in Kissimmee, and so knows the area inside out!
Always make sure you can get a receipt for your payment, and get a booking form with the cancellation details, and information about the security deposit stated on it. And if you book direct with the owners, make sure you have a way to contact them while you are away, and not just the management company.
Personally, we are available via phone, fax, email, web site (and post if necessary), and are happy to deal with any queries or requests, no matter how trivial you may think they are. Before we bought our home, we stayed in a few villas, with differing levels of contact with the owners (yes, the lower service was with a US owner!), so we were determined to give as much information and advice to people, and be available just as much as the management company!
You could also try
www.greatrentals.com. They have over 10,000 homes across the world, not just in Florida (and yes, we are listed in there!).
I would advise certainly to try to speak to the owners before booking, you get a better idea about them, and they also get a better idea about you (not everyone is considerate about other peoples homes)! If you get any problems, a direct method of contact and a name is so much more reassuring!
Hope you find your perfect holiday home!