Three providers that I just can't recommend enough:
http://www.paulmarkham.com
The content at Paul Markham gets moved to:
http://www.bargainbasementcontent.com
When it's been on the Paul Markham for a while, but BargainBasmentContent is a great site for quality filler content.
I also like the stuff at Richard and Jeremy's content store:
http://www.adultcontent.co.uk