Since you can't buy it from the source, your options are limited to the secondary market.
A genuine boxed copy with a holographic disc and a COA sticker is the gold standard for authenticity.
The short answer is:
The internet is full of "discount" keys for $5. While these often activate, they are frequently "gray market" keys—meant for educational institutions or large corporations—and can be deactivated by Microsoft at any time without warning.
You might find System Builder or OEM keys on sites like eBay or through smaller tech vendors. These were originally intended for manufacturers to pre-install on hardware. buy original windows 7
Are You Using a Legal Copy of Windows? - Business Owner Stories
One of the most reliable ways to get a legal license is to buy a refurbished "Business Class" laptop (like a Lenovo ThinkPad or Dell Latitude) that still has the Certificate of Authenticity (COA) sticker with a valid 25-digit product key attached. Red Flags: What to Avoid Since you can't buy it from the source,
While Windows 7 can still be installed and activated today, remember that it no longer receives security patches. If you do set up a "new" Windows 7 machine, it is highly recommended to keep it offline or use it strictly for offline tasks to avoid modern security vulnerabilities.