Change en-us to your local language and country if necessary, but https://support.microsoft.com/en-us/help/15045/windows-anytime-upgrade-not-available says:
"Windows Anytime Upgrade is no longer available for online purchase in your country or region. To move up to an edition of Windows with more features, visit your local retail store or visit Windows.com."

Since you didn't tell where you live, nobody here knows where your local retail store is. However, amazon.com might be a good alternative (I didn't check).
Since you can download the installation disc, https://www.google.com/search?q=windows+7+license+key might lead to something also.