Mike,
Boxes from the following stores are good: guitar, furniture, bicycle. Build the boxes inside-out, taking advantage of existing fold lines where possible, otherwise cutting through one layer on the fold lines. A heavy duty stapler is a good idea, like a Rapid HD31.
Or, if you want to do the job more quickly, buy large sheets of cardboard from a local box-maker. I buy 79" x 54" sheets at about $4, enough for several balsa or kit boxes. In my region, the firm is Diamond Container.
Peter