to get a render without a background go to www.render-world.com
and go to gallery. then pick a subcategory, then when the big list comes up with the render previews, find one you like and click the thumbnail for it. then in the page that loads, you will see the r3edner. click it again. that will open in a new window and you can then go to file>>save as>> PNG. iof it wont open in a new window wen you click again, just save it from there.
this also works from www.planetrenders.net but you must make a free account before youc can get the fee background-less renders