You might use PLR material.
Do a Google search for "PLR [your topic]".
This will locate products that offer their "Private Label Rights" for sale. You can then revamp, reformat, and rename those PLR products and make them your own. It's perfectly legal -- that's what the PLR seller INTENDS for you to do.
For instance, if you were selling an ebook about Dog Training, you could search for "PLR dog training". I just did that search and found...
http://www.petniches.com/
http://www.articlebrainstorm.com/ (collection of 300 articles)
http://dogbooksmonthly.com/
Now you simply re-package some of this material and offer it as you bonus material.
You could offer three additional ebooks, for example. Or one additional ebook, plus a one-year subscription to your dog newsletter. (With the 300 articles, for example, you could just pick 52 of them, load them in your autoresponder, and VOILA - you have a one-year newsletter on autopilot!)