Most computer stores will still have windows 7 cd's, but you can't buy just a cd key. Why would you need a cd key, but not the cd? If you already have the disk, then there should be a key somewhere (on packaging, in packaging etc.) Unless you have a copied (illegal) disk.
In that case, you may as well just buy a fresh disk, because windows is clever, and it knows when it's a copy.
But any computer store will probably still have win7.