For personal use, buying and using an OEM key is generally considered "grey area" but not illegal for the end-user. Microsoft's EULA (End User License Agreement) technically states OEM keys should be sold with a fully assembled computer system, but they rarely, if ever, penalize individual home users for using them on custom builds.
You can find OEM keys through various channels, ranging from official distributors to "grey market" resellers. buy windows oem license
: Marketplaces like VIP-SCDKey , Keysfan , or Kinguin sell digital OEM keys at deep discounts. These are often "grey market" keys—surplus licenses from other regions or high-volume buyers. While they usually work, there is a small risk of the key being deactivated later if Microsoft deems it a violation of terms. How to Use an OEM License For personal use, buying and using an OEM