Official sites like Microsoft are the safest but most expensive route.
Keep in mind that Microsoft will end official support for Windows 10 on October 14, 2025. After this, you may need to pay for Extended Security Updates to keep the system secure. How to Download Windows 10 For Free (Official) cheapest way to buy windows 10
If you have an old laptop with a Windows 7 or 8.1 sticker (or a digital license linked to your Microsoft account), you may still be able to activate Windows 10. Official sites like Microsoft are the safest but
You can install Windows 10 without a product key. You will get a fully functional OS, though you'll face a persistent "Activate Windows" watermark and lose access to personalization settings like changing your wallpaper. How to Download Windows 10 For Free (Official)
If you want a full activation for the lowest price, third-party marketplaces often sell keys for a fraction of retail.