Someone had already posted earlier:
Where to buy a ROC key?
Although this thread was made two years earlier, when Reforged didn't include a single player functionality at the time when it was released, you can still play single player (custom games or LAN) nowadays with Classic graphics.
You can also add your CD-Key to your account in the Blizzard site, then redeem your code.
Last, but not least, please read BogdanW3's notice if you want to play Reforged with only ROC CD-Key:
To get 1.32 with just a ROC cd-key (as TFT had become useless after 1.30.2), go to the Blizzard site and download the installer from there, the launcher only semi-supports the cd-key method.