Best place to buy Win 10

[jonm01]

Will be installing Parallels with Win 10 on a new iMac, where's the best place to get Win 10? There are a lot of sellers on Amazon selling keys for circa 10 pounds, are they legit?

[jonm01]

Bought a Win 10 Pro license from an Amazon seller fo £8. It worked fine.

[jonm01]

Isn't that just for upgrading?