The category of content you are referring to is called “PLR” (or private label rights). A quick google search returns a few sites that let you can “license” the content and rebrand them as your own. Here’s the first one that came up on my search:
http://www.plrprivatelabelrights.com/
You should probably consult their terms of use and an attorney as well.
Additionally, I highly advise you to read the content and make modifications to any PLR you choose to use and tailor it for your intended audience. You can assume that there already are lots of copies of them floating out there in the wild, so some differentiation would only benefit you.
Where is a site that i can buy ebooks from and then LEGALLY re brand them?