Make one.
1) dismantle typewriter and save the keys
2) dismantle keyboard and remove keys
3) make a hollowed out 2 part wooden case for the keyboard
4) fit the typewriter keys in to the empty slots from the keyboard keys (the slots are probably square)
5) Assemble your new case

It isn't that simple, especially if you don't have proper woodworking tools, but it isn't the most complicated project in the world either. If you are in high school then I suggest taking woodworking. Maybe I got lucky, but my teacher allowed me to make whatever I want. While everyone else was working on a mousetrap car, I was making a case for my ZuneHD.