Answer:
The most common bootable device around is the hard drive. That is if its formatted.
So lets say you have a clean hard drive with nothing on it. Well get to your nearest computer and format it and install a new OS on the hard drive.
You dont have to buy expensive ones since there are many free OS like Ubuntu or Fedora etc etc.
all you have to do is format a CD to install the OS image and install it in your hard drive.
Also a CD and a DVD disc can be bootable even a portable USB all you have to do is format them so they can be able to boot.