As of July 1st, 2018,ServiceNow requires the official ServiceNow training as a prerequisite to the exam and certification exams cannot be purchased directly.
These is very unfortunate because I know how expensive ServiceNow's training is.
ServiceNow, y tho?
An alternative to this desperate direction by ServiceNow is to ignore the certification and prove your skills during an interview. Many ServiceNow partners are more than willing to cover your training and exam costs, since their partner tier is tied to the number of employees who hold accreditation.
The whole system of certification is flawed if you ask me. Anyways, I hope this helps clear some confusion 🙂