You may be able to find a couple of ebooks that you can purchase cheaply with mass-resale or distribution rights, but there is a reason that these ebooks are cheap: they are of low quality. Well though out, quality eBooks take time to create, and their authors will charge accordingly.
To find a writer to create an ebook for you as as bonus offer, you can post your project to a freelance site, like elance.com and see what bids come in. Writers should provide examples of their work, or direct your to an online portfolio, to help you make your choice. It may cost more upfront, but your customers will appreciate it and be more likely to buy from you in the future.
Offer training
If the product you are selling requires a learning curve (like computer software), then an effective bonus is to offer a training incentive. This could include an instructional ebook (either write it yourself, or find a freelance writer as described above), online support forums, or critiques of people's work.
For example, let's say the product is a step-by-step guide to getting started with affiliate marketing. Well, you probably have more experience in that field than the person buying the book, so offer to look over the websites and offers they create, and then give them feedback. If you do a good job and they make some money, then they'll probably be interested if you offer them a product about intermediate or advanced affiliate marketing skills.
Keep in contact
One downside to internet sales over person-to-person sales is that customers may feel disconnected from the person they are buying from. When you buy a TV at a store and something goes wrong, you can go back to that store and get some answers, and that is very reassuring to most customers.
If you make yourself available, either via an email list, support forum, or some other means, then customers will see you as a person, and not just a website. The personal touch in the internet's impersonal world can be the key factor in getting you sales.
