What is powering your website right now?

If you use a content management system, there may very well be a plugin that you can install that can do this, but it will depend on the system you have. If this is a custom website, all you would need to do is add an intermediary webpage between the download button and the checkout to collect that user information. That new page would contain a form that, upon submission, would direct the user to the checkout. Also, you will need a privacy policy that explains what you will do with this information. Right now, I only found a terms of use.

If you're open to something entirely a new, you can take a look at a content management system like Drupal or Wordpress where you can accomplish something like this as well.

Lastly, if you're looking for developers to help you implement this, I suggest putting your project out for a bid on one of those freelance websites like freelancer.com. You could also look locally in your city on LinkedIn or Twitter for developers in your area. It often helps to have the ability to sit down face to face.

~Sovereign