Thanks, everyone. These are wonderful ideas to keep in mind! Not just for my math club kids, but for my own kids as well!
We don't have a homeschool bookstore here (there used to be a school supply/teacher resources store but it was long gone). So I rely on online stores mostly.
I love the Dover books myself. The Moscow Puzzles book, interestingly, is the one I had in mind if I can't find anything cheaper. It's a really fun book. Last year I gave the math club kids a Dover puzzle book (was able to buy it when amazon.com had a sale), and the kids loved it. Some read it from cover to cover and told the new kids in the math club about it. So the kids are looking forward to it. :-)
