Most folks on this board will point you to the big VRBO sites...like:
www.vrbo.com
Or, there's sites specifically for the various resorts to rent directly from the owners:
www.windsorhillsrent.com
Windsor Palms
Emerald Island
Indian Creek
Here's a map of the offsite resorts in the area:
http://www.thedibb.co.uk/villa/map.htm
Those mentioned above are probably the closest, newest, and nicest (but I'm biased as we own townhomes in Windsor Hills). They are STR rental communities (meaning no one really lives there full time) with full resort facilities, pool, clubhouse, gated communities, etc, and they're all 5-10 minutes from Disney, as close as staying on the WDW property but you get a lot more for your $$!
You can rent some of the exact same properties you might find on ebay, and probably have a much better selection too. The various ad sites will have links to the owners private websites where you can check out the properties even further. There you can review their online guestbooks, etc. As the prior posters have mentioned, having a local property MC is VERY important! Equally important is having a GOOD local MC, one way to assess this is by looking at past reviews of the property, but you should also ask the owner who their MC is, and a good question to ask is how long they have been with their current MC (because if they're bad chances are the owners will fire them!). There are a few MCs I wouldn't rent from, and even if the home or the owner were great, I wouldn't stay there, period, I've been privy to "Horror" stories from other owners and guests being an owner.
You can also find reviews from DISboard members who have stayed in many of these various properties as well, click on the reviews boards up above!