Sometimes it's best to create the ebook yourself. A lot of these sites offering reselling rights have products that are ummm... less than stellar as ppcfanatic said. Or there are products that can be found anywhere.
Allan Gardyne's free ebook that he gives away when you sign up for his newsletter subscription on
http://www.associateprograms.com is a great example of truly offering something of good value. That book is totally worth an email address.
I've used a couple of his techniques in that book and it has improved my revenue from my sites.