Hello,
Unfortunately, you aren't able buy product keys for Windows 7 (for any edition) anymore, since Windows 7 went out of service.
If you still have your OEM product key (usually in a CD or on a COA on your laptop/desktop) or on a retail (box) copy, you might still be able to use that, but aside from that, getting Windows 7 activated is going to be hard.
If you can, you should attempt to upgrade your PC to at least Windows 10 so you can have an activated system.
Hope this helps.