Mathematics papers can be published by a university journal, a private organization or independently. The most reputable math papers are published in peer-reviewed journals. Within the United States, a popular journal is the Journal of the American Mathematical Society, or JAMS.
Many reputable mathematics journals, such as JAMS, have a steep subscription fee. Because of this, the best way to access subscription-based journals is through libraries. Checking online or calling is a good method to see which journals a particular library carries. Google Scholar can also be a useful tool for finding specific math papers, allowing searches based on academic field and citation count.