Unfortunately, there is no official Cisco CCDE practice product. However, I would highly recommend checking the following learning resources, if you haven't already done so.
First, we highly recommend reviewing the full list of exam topics and utilizing them as your guiding map while you prepare for the CCDE Written Exam.
Then please feel free to review the CCDE Written Exam Study Material page for a handful of resources and recommended training to help you prepare (book list, training course options, online resources, etc.). One very helpful resource in particular is the Learning Matrix document. We also have free on-demand videos that you can access here on the site: Cisco Certified Design Expert (CCDE) Training Videos
I would also highly recommend checking out the Unleashing CCDE blog series. The blogs are written by various experts and guest contributors discussing network design technologies and providing guidance, tips/advice and more.
We wish you luck on your certification goals!
Cisco Learning Network Moderator
Sorry, but I don't have a suggestion for buying practise questions for the written.
However, some people I've studied with have made their own by using their favourite online flashcard system. They create questions and answers while reading/studying and then use the flashcards for review.
https://quizlet.com/ seems to be a popular platform.