Where can I get Windows 10 product keys to upgrade my Windows 7?

Currently there is no information available regarding how to upgrade from Windows 7 to Windows 10 for free. But whenever I get some information on it, I will surely update this thread.
In my opinion, 1 year of free upgrade of Windows 10 is like a trial version of any software, so I don't think we will get any product key of Windows 10. Instead we need to upgrade Windows 7 to Windows 10 via Windows Updates.

As per the reports available from some sources, Windows 10 will be offered as an update via Windows update. Windows will see it as a normal update and install it. If you have automatic updates enabled, Windows 10 might get installed without the user having to do anything.
Please note that you will not be able to upgrade from 32 Bit Windows 7 to 64 Bit Windows 10. It is not yet confirmed whether Microsoft will release 32 Bit version for Windows 10.
Also note that if you upgrade to Windows 10 within one year from the launch, you will not need to pay anything.