I need to ship (FedEx) my keyboard (Technics KN5000). Can someone recommend WHERE I can find a heavy duty 'cardboard' shipping box?

The Keyboard's dimensions are:
41-7/8" long x 16-3/8" deep x 7" high
106.2 cm long x 41.4 cm deep x 17.4 cm high

I'm wondering if cardboard box manufacterers or packaging stores even carry cardboard boxes with dimensions to fit our keyboards. Anyone have a SPARE cardboard shipping box which will fit my keyboard?

- Scott