I don't know the AoPS MathCounts course; my son took the AMC 8 advanced prep course a couple of years ago and enjoyed it enough to want to take both AIME courses this summer.

There's a MathCounts Coaches' Forum on AoPS (go to 'Community'; then click on 'Middle School Math'); you might want to post your query there.

My son's teacher and coach use problems from various sources:

old MathCounts contests at all levels, obviously, accessible via the OPLET service or in book form in the online store at mathcounts.org, *if* you've got the money; assiduous Googling can also net you some for free

the various textbooks published by AoPS - Intro to Counting and Probability, Intro to Number Theory, etc.; again, not cheap

Jason Batterson's 'Competition Math for Middle School', available on Amazon for $25

old problems from other competitions; you might want to browse through (and Google from) this chart: http://homeschoolmathcontests.com/page.php?6

Hope this helps -- good luck!