Where can I get test samples for Assistant Air Traffic Controller Officer exam?
Based on the country you are looking for the certification, rules may be slightly changed. For India following syllabus is applicable (Source: atcguild.com)

At present it is a year-long ab-initio training (Under revision to make it in two sections of six months each)
Modules:


Air Traffic Services
Aerodromes and Ground Aids
Air Legislation
Meteorology
Communication Procedures
Technical
Search and Rescue
Air Navigation


The selection shall be made on the basis of a


Written test
50% questions on subjects relating to educational qualifications
prescribed
50% on General Knowledge, General Intelligence, General Aptitude,
English etc. Voice test
Personal Interview
Medical fitness as per ICAO Annex-1


I Googled for the subject, but didn't find much information on the paper patterns or example papers.