Unfortunately, you cannot just buy the chassis. You must buy a barebones notebook that comes with the motherboard, graphics chip and chipset. The CPU, RAM and HDD are empty for you to configure.
The reason why is because notebooks don't have standards like ATX or Micro ATX like desktops do. So if you buy a notebook chassis, it must be barebones with the motherboard. The motherboards are designed for the shape and size of the chassis itself, which means there cannot be any set standard for it.