Here are some links for practicing the types of problems you'll see in MathCounts or on AMC 8.
MathCounts Problem of the Week: Every week, a new multipart problem related to a holiday, season, special event or cool STEM topic is released. The answer is posted the following week within the problem itself (in blue).
MathCounts Minis: Minis provide detailed explanations for MATHCOUNTS problems and their associated math concepts, plus include activity sheets.
MathCounts Practice Plans: Begins with a warm-up problem to practice related skills, then is followed by a video lesson, a bunch of practice problems, and finally a synthesis type problem to extend the topic.
MathCounts Trainer: located on The Art of Problem-Solving's Website, and is developed for those students that find they need to be challenged in a more rigorous way than most classroom settings. The Art of Problem-Solving has a bookstore, online classes, and other resources available.
AMC Resources consists of materials for teachers, parents and students to help with mathematical problem-solving and to prepare for the MAA American Mathematics Competitions (AMC).
Brilliant is a collection of lessons for students to help them learn many of the concepts used throughout the year in Math Counts.
Neither OPUSD nor I, as an individual, recommend that club members spend money on purchasing any materials unless they get their parents' approval first. If students/families choose to purchase any material, it's for their personal use and not a part of the MathCounts Club.