Yes, you can get an original bootable disk from a good computer store, all you need to do is just ask for the windows edition you are looking for, or you can simply buy an installation disk from any of the computer stores and then buy the key to activate the windows. If you are using a genuine window at present then no need to buy an another one, just put in the same key and your windows will be activated.
Note :- You can use the same key only if you install the same windows as you was using before.