Is this an upgrade from from home to pro on a computer that originally had 8 or 8.1?
You can download the same version as what 8.1 version you had here. If you computer is OEM with 8 or 8.1 preinstalled you will automatically be activated. Just skip the windows that asks for the key. If you are upgrading from home to pro you will need to purchase a Pro key.
I have no experience with the company but you can buy an OEM disk if you plan on keeping the OS with the computer.
You can go the Ebay route but make sure the sell has a high rating and you can receive a refund from ebay if the sale is not legit.