Hi!
You can find official resources in Nintex Learning Center: http://learning.nintex.com/webapp/#/course/73 (link to help for the admin certification you mentioned).
There is also training area in this forum: https://community.nintex.com/community/training. Or just google. Maybe on Pluralsight?
Nevertheless the best way to get knowledge to pass the exams is just to practice. They're not that hard after all
Regards and a happy New Year,
Tomasz
There are a lot of good documents out there, for the certification these two helped me a lot:
Database Design Guide: Nintex Workflow
https://community.nintex.com/docs/DOC-1426
NWAdmin Operations - Nintex Workflow 2013
https://community.nintex.com/docs/DOC-1026