Talk to your contractor. If you want to put Keratuer doors in a new house that's being built, the contractor will be able to order doors directly from the manufacturer. You can use the Keratuer website to pick the style and type of door you want and the contractor can design the room based on those specifications.
Use one of the official Keratuer partners. The Keratuer website lists companies that sell Keratuer doors, including website information, telephone information and general contact information. All of these companies are based out of Germany or other locations in Europe, so you may have to find someone who speaks German (or see if the company can provide someone who speaks English) to assist you in ordering.
Use the "Contact" form on the Keratuer website to contact the company directly. Input your name and contact information as well as your reason for writing. You can purchase and import these doors directly from the manufacturer, but note that all other factors considered equal, they will cost more than a door purchased domestically.