Many older Windows 7 and 8 keys can be used to register a copy of Windows 10 Professional (with varying levels of success), but if you don't have one laying around, it's now less money to buy a new key. Woot (a store owned by Amazon) is now selling Windows 10 Pro discs for $99.99, a savings of $100 from the regular price (opens in new tab).
Woot originally had both USB drives and DVDs for installation, but the former has sold out already. The product key is inside the disc sleeve, so you don't actually need a PC with a DVD drive.
Even if you already have Windows 10 Home (the version that ships with most pre-built systems), it might be worth making the jump to Windows 10 Pro. There are a few features exclusive to the Pro version, including Remote Desktop, BitLocker full-disk encryption, and Hyper-V virtualization. Here's a full breakdown of the difference between Windows 10 Pro and Home.
You can buy a copy from the link below. The sale ends today, or whenever stock runs out.
Microsoft Windows 10 Professional | $99.95 ($100 off) (opens in new tab)
If you don't have an older Windows key lying around that can be used to activate Windows 10, this is a good deal on a 100% legit key.