School math books for sixth graders can be found online. AbeBooks and Barnes and Noble have such books for purchase.
The University of Chicago (School Mathematics Project) provides a literature list for 6th graders that mathematics teachers can use at schools. Books such as "Math Curse" by JonScieska and "Counting on Frank" by RodClement are part of the university's list. The first book, which explains about geometry with illustrations and has won various recognitions, can be purchased through Barnes and Noble. The second book, which explains about number and order, can be purchased through AbeBooks. AbeBooks acts as a link to various bookstores and, thus, provides different prices for the book and other school math books.