Below are some suggested materials for students looking to do preparation problems ahead of the Stevens Math Olympiad.
During the coronavirus quarantine period, the National Museum of Mathematics is providing a new program, Mind-Benders for the Quarantined!, to keep your minds fresh! Participants will receive a challenging mathematical puzzle each Sunday.
The Mathematical Circles Library, which contains a number of volumes on problem solving for various grade levels. These books are not free, but many are reasonably priced.
Brilliant provides free math fundamentals and competition preparation courses. Sign up is required.
Problems and solutions from the American Mathematics Competitions. These problems are for grades 8-12 only but they are freely available. The website also links to further resources for preparing for a mathematics competition.