You should try looking deeper through Microsoft's website and see if you can buy one through there.
You can always also do a search for something like "Buy Windows 8 64 bit product key"
I did one and I actually found a product key available to buy on Amazonhttps://www.amazon.com/Windows-Pro-Upgrade-Product-Card/dp/B00A6TPHZS
Although I'm not quite too sure if it's legit
Once you've gotten a product key, just download an ISO image of Windows 8 64 bit and install it on your computer with a USB.
During the setup, it may ask you for a product key, I believe in 8 or 8.1, one of them, you can skip the product key inputting, but when you're in Windows, the system will every few minutes or after an hour or 2 keep bragging you to activate windows and what not.
If you got a key, just put it in and you're done!
Oh, and also, yes, you can upgrade to 8.1 when you're on Windows 8, for free too, I think. Should be, I think. Once again download the iso for 8.1 64 bit and run the setup and let it upgrade you