There are 2 main companies which produce them, Examcraft and DEB (Dublin Exam Board). Most teachers will choose exams from one of those. However some teachers will set their own. And there are a number of schools which don't do 'Pre/Mock' exams at all. After all, it is the schools choice.
The most important 2 things to remember are:
1) They are just as important as a practice match before a final. The result isn't important, but what you learn from the match is.
2) there is absolutely NO LINK between the 'pre/mock' exams and the state exams in June.