We hate not having Apple's Calculator on our iPads and while there are some great apps out there why don't you go old school with our new Abacus Case! The case has ten slots with 10 counters in each that slide along nicely. You can buy it or print it in one or buy or print in parts and assemble it.
You can have your own case with custom colours and materials by buying the frame case and the plugs separate.
Plugs (You need ten sets): http://www.shapeways.com/model/885205/ipad_mini_abacus_case_plugs_10x.html
More info here: http://gromfrog.com/?p=1647