I'm not gonna answer about the best place, I will give you a more general answer.
Virgin and PcAndParts sell Windows. (Keys,dvds Not sure)
You can also buy a key online directly from microsoft or key resellers.
Or the cheapest one, from someone selling MSDN keys on forums. You can get a windows key for around 5$
Anyone can link me to where i can buy msdn keys
I bought a win7 key from Reddit
https://www.reddit.com/user/s5ean

The seller is a reddit moderator and a legit member, I upgraded to win 10 pro around 3 months ago and been working great.

I paid him with skrill.
16 days later
Just a little update. I bought a windows key from s5ean and it's running perfectly.
lallous_87 wroteJust a little update. I bought a windows key from s5ean and it's running perfectly.
Awesome, glad to hear :)
Just a small question, where does he get his keys from? why are prices so cheap? 20 $ for a windows server?
ironman wroteJust a small question, where does he get his keys from? why are prices so cheap? 20 $ for a windows server?
Not quite sure, but I remember I read once he has access to retail Microsoft products..
Hybrid wrote
ironman wroteJust a small question, where does he get his keys from? why are prices so cheap? 20 $ for a windows server?
Not quite sure, but I remember I read once he has access to retail Microsoft products..
How did you pay him Hybrid ?
Aly wrote
Hybrid wrote
ironman wroteJust a small question, where does he get his keys from? why are prices so cheap? 20 $ for a windows server?
Not quite sure, but I remember I read once he has access to retail Microsoft products..
How did you pay him Hybrid ?
Skrill, if he's not displaying it as payment option, just message him, he will give you details.
2 months later
@Hybrid and lallous_87, did you guys use a special windows 7 provided by him, or any windows 7 would work?

Thanks
ironman wroteJust a small question, where does he get his keys from? why are prices so cheap? 20 $ for a windows server?
No idea but it's probably an OEM version, they are cheaper and meant to be sold to manufacturers. And OEM Windows can only run on one machine and if your machine breaks or becomes too slow and you need to move to a new one then you have to buy the OS again (unless you call Microsoft).
i got my windows 8 pro key from G2A around 8months ago for ~15$ and also used it to upgrade to Windows 10.

It seemed sketchy and I didn't really have high hopes of it working at all but it did and never had any problems
I prefer to use Linux. The only reason I'd use Windows is for Adobe Photoshop. I don't play any PC games.
Granted, it's not for everyone.
ironman wroteJust a small question, where does he get his keys from? why are prices so cheap? 20 $ for a windows server?
I read on another forum something about MSDN keys, no idea what that is, but it's working perfectly fine
ironman wrote@Hybrid and lallous_87, did you guys use a special windows 7 provided by him, or any windows 7 would work?

Thanks
I used a link he provided on reddit, no idea where the image was hosted, but the upgrade was done smoothly, and later I formatted and re-installed windows 10 that I've got from another source, and everything is still working fine.
@Hybrid,

Well my problem is that i installed W7 from one of those DVD's (3000 LL) and i thought about buying a key from him to activate, seems it won't work...
Thank you for your feedback
i can sell u a genuine key for 50 $ pm me
MrElie wrotei can sell u a genuine key for 50 $ pm me
Why buy for 50 when i can get it for 25 :p ?