Not a sales channel - where to buy Windows FAQ
The Microsoft Store is the simple answer.
We're not a sales channel but this question gets asked occasionally so here's a faq to keep it out of the channel.
Where do I buy Windows?
Legitimate Windows tends to be the same price regardless of sales outlet you choose so really, Microsoft Store, Amazon, NewEgg, favorite retail or online store in the country you reside in.
But I found this 'random site/ebay/whatever' where it's cheap.
Remember, if the price seems to good to be true, it probably is. Someone is probably reselling keys in the grey/black market. Things like this lead the ending of various TechNet programs in the past so let's not support them or chat about them in channel. We're not lawyers or sales people so we're not going to vet them or drive traffic to them.
But it's expensive and I'm poor, etc etc.
As a reminder, we have no humor about piracy or suggestions about piracy. however...
Windows 10 can be used unactivated. It is the Home version only, but it will not expire and you can install and run regular programs and apps.
While there are multiple different SKUs, you can really only purchase Windows 10 (Home) and Pro.
The Windows 10 Education version can be obtained through some school programs (see your school to determine if you can access these programs).
The Windows 10 Enterprise version can only be obtained through a VAR with an SA (Service Agreement) contract.
Windows 10 S is sold only to OEM/Hardware vendors as far as we can tell.
For the consumer OS version, there are two different types which affect price. You need to review the license and determine which suits your needs and skillset to determine which to purchase.
From howtogeek site's summary
Full Version/Retail licenses of Windows are the standard “consumer” version of Windows. If you’ve ever walked into an electronics store and saw a boxed copy of Windows on a shelf, you were looking at a retail license of Windows. These are designed for sale to average computer users who may be buying a new Windows license to upgrade their computer to a new version of Windows. It allows them to take their copy of Windows and install it on any PC they like — but it can only be installed on one PC at a time.
System Builder/OEM licenses of Windows are used by computer manufacturers — “Original Equipment Manufacturers.” Not only are they used by large computer manufacturers like Lenovo, Asus, Dell, and HP, they’re used by the local computer shop you might buy a custom-built computer from. Microsoft has swung back and forth on whether “enthusiasts” can use System Builder licenses of Windows when building their own PCs. This type of license is designed to stay tied to a single PC forever.
Finally, just buy the current version of Windows. Really, the older versions are in extended support and just getting security patches and Microsoft's focus is on Windows 10.