Past papers are generally available from the examination board or institution that sets the exam; this is the case for many math exams. Math SAT papers are available online from a variety of study help sites including Careers 360, ProProfs and the official SAT site run by the College Board.
Many sites that offer practice SAT papers allow users to take the tests online for free. Several of them even allow downloads of the questions. However, some sites require users to register before they take the tests, and they may ask for permission to send marketing-related questions concerning exam techniques, training and further study. Many of the sites are unofficial and unendorsed by the College Board; however, the questions on them are taken from genuine past papers, so they are still helpful for study purposes.
In terms of preparing for other math exams apart from SATs, it's important to go to the source of the paper and seek the advice of the institution that sets the exam. Most colleges and exam boards have websites that give applicants advice about how to prepare for exams and where to find the most recent available past papers. Universities have extensive resources for students to access while preparing for exams, which may include special mock papers, as well as copies of past exams.