I bought Windows 8 Pro in Microsoft Site and i have a Windows key.
My Windows 8 is 32 Bits but i see now that my computer supports 64 Bits.
How and where i can download Windows 8 Pro 64 Bits with the same Key, to install and test it?
When you download a Windows installation it automatically matches the existing install so you would get 32 bit download on a 32 bit machine. Only way I can think you'd do what you're aiming for is to download windows 8 to another pc which currently has 64 bit Windows 7 or Vista installed. Burn a disk from the iso download and install to your pc.
To add to what patcooke said, there is unfortunately no legitimate direct links on the web as there is for Windows Vista/Windows 7, so running the Upgrade Assistant from a 64-bit PC is your best bet. It will offer to create and burn an ISO to a disc or it will let you create a bootable flash drive to install from (so long as the flash drive is 4 GB or more). Peronsally, I always chose the latter as it makes the installation go so much faster (read speeds are higher from a USB drive than a DVD).